Fortschritte der Physik / Progress of Physics: Band 25, Heft 9 1977 [Reprint 2021 ed.] 9783112519523, 9783112519516

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FORTSCHRITTE DER PHYSIK H E R A U S G E G E B E N IM A U F T R A G E D E R P H Y S I K A L I S C H E N

GESELLSCHAFT

DER DEUTSCHEN DEMOKRATISCHEN

REPUBLIK

VON F. K A S C H L U H N , A. L Ö S C H E , R. R I T S C H I . U N D R. R O M P E

H E F T 9 • 1 9 7 7 - B A N D 25

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K

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V E R L A G

EVP 1 0 , - M 31728

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Zeitschrift „Fortschritte der Physik" Herausgeber: Prof. Dr. Frank Kaschluhn, Prof. Dr. Artur Lösche, Prof. Dr. Rudolf Ritsehl, Prof. Dr. Robert Rompe, im Auftrag der Physikalischen Gesellschaft der Deutschen Demokratischen Republik. Verlag: Akademie-Verlag, DDR - 108 Berlin, Leipziger Straße 3 — 4; Fernrul: 2 23 60; Telex-Nr. 114420; Postscheckkonto: Berlin 350 21; Bank: Staatshank der DDR, Berlin, Konto-Nr. 6836-26-20712. Chefredakteur: Dr. Lutz Rothkirch. Anschrilt der Redaktion: Sektion Physik der Humboldt-Universität zu Berlin, DDR - 104 Bertin, Hessische Straße 2. Veröffentlicht unter der Lizenznummer 1324 des Presseamtes beim Vorsitzenden des Ministerrates der Deutschen Demokratischen Republik. Gesamthcrstellung: VEB Druckhaus „Maxim Gorki", DDR - 74 Altcnburg, Carl-von-Ossietzky-Straße 30/31. Erscheinungsweise: Die Zeitschrift „Fortschritte der Physik" erscheint monatlich. Die 12 Flcite eines Jahres bilden einen Band. Bezugspreis je Band 180,— M zuzüglich Versandspesen (Preis fiir die DDR: 120,— M). Preis je Heft 15,— M (Preis für die DDK: 1 0 , - M). Bestellnummer dieses Heftes: 1027/25/9. (c) 1977 by Akademie-Verlag Berlin. Priiited in the German Dcmocratic Repuhlic. AN (EDV) 57618

Fortschritte der Physik 25, 5 1 1 - 5 7 8 (1977)

Radiation Damage Products in Ionic Crystals Impurity Doped Alkali Halides S. RADHAKRISHNA a n d B . V . R .

CHOWDARI

Department of Physics, Indian Institute of Technology, Madras,

India

I. Introduction Several hundreds of papers dealing with radiation damage in solids in general and impurity doped crystals in particular are scattered in literature and we believe that it is the time to consolidate and assimilate this vast amount of literature and information available if further work is to yield meaningful progress in the right direction. I t is the purpose of this article to focus attention on the different kinds of radiation damage products and the experimental tools t h a t have been put to use to unravel the physical properties and models of such products. Emphasis will be laid on impurity doped crystals. We do not include metals and semiconductors in the present survey as the nature of products and the materials are of a very different nature. Quite a few experimental techniques have been exploited with a view to gain as much information as possible on the radiation damage products. Undoubtedly, magnetic resonance (ESR), optical absorption and luminescence have played a dominant role in this process with support coming from measurement of volume and lattice parameter change, magneto-optics, ionic thermocurrents, thermally stimulated currents, pulsed electron beam techniques, and flow Stress measurements. For the purpose of creating damage, sources of ionising radiation ranging approximately from 5 eV to 30 MeV have been used. Depending on the nature of the material and the extent of damage required, UV irradiation 5.0 eV), X-irradiation 50 keV), y-irradiation 1 MeV), neutron irradiation 2 MeV) and electron irradiation (1 — 30 MeV) have been used. Several excellent reviews are available [i—6] on radiation damage in solids but in most of these articles the emphasis has been more on the mechanism of damage and radiation products in pure crystals and the experimental tools used in their study. I t is therefore the intention of this article to present a comprehensive picture of each radiation product in as complete a manner as possible, discuss the role of impurities and above all present all the damage products in impurity doped crystals at one place. For the purpose of this survey we have categorised the various solids into different types based on t h e similarity of the end products and on the materials themselves.

II. Radiation Products in Alkali Halide Crystals I t has been conclusively established that when a crystal belonging to the NaCl type or CsCl type is irradiated, both interstitial centres and [4, 7—10] vacancies are formed as complimentary pairs. Such pairs have been categorically defined and identified by 38

Zeitschrift „Fortschritte der Physik", Heft 9

S. Radhakrishna and B. V. R. ChoWdabi

512

radiation damage and subsequent experiments at low temperatures. The vacancies yield electron excess centres when they trap the electrons produced in the process of damage and interstitial entities, when stabilised, usually give hole excess centres. In Fig. 1 a schematic sketch of such radiation products is given. We will begin the discussion by presenting the maj or features of electron excess centers and the influence of impurities on such centers. So much has been said about the F center, its production and its properties that it is not relevant, in the present content, to discuss its details. The major part of the article will be devoted to radiation products involving impurities. Electrons

HaLiâè~\Clectron F-Center Cl °^ÎjCaptwrë

Halite

y*x~

aggregate centers

c

ionisation or excitation

T°K

Haggrega*e centers

F -Center

Vk-Center

Interaction with X'ion

cc-Center

Interaction with 3X~ions

Electron

Capture

I-Center

H-Center

aggregation

Fig. 1. Radiation damage schematics. The blocks one below the other indicate complimentary pairs. Blocks of dashed lines indicate transient products. The products are arranged at different levels based on their relative thermal stability

I I . 1. F center A major product of radiation damage in the NaCl as well as CsCl type of crystals is the well known F center, which consists of an electron trapped at a negative ion vacancy with the electron spending approximately the same time on each of the surrounding positive ions. I t displays a bell shaped absorption band, the peak of which depends on the lattice constant of the material, gives a prominent emission band at a different wavelength, gives a fingerprint like electron paramagnetic resonance signal, characteristic thermoluminescence, besides showing itself up in several experiments [J—6]. However, in the case of cesium halides [11, 12] the F center optical band exhibits a structure which has been ascribed to the spin-orbit coupling. While the peak position of the absorption and emission bands, and the E S R signal are independent of the nature of the ionising radiation, the intensity of these features and the efficiency of radiation damage is dependent on the radiation dose that the sample is exposed to, the temperature at which the damage is caused, the purity and the previous history of the sample. We will briefly summarise thè results of radiation damage and subsequent formation of F centers at room temperature before examining the influence of these parameters. Most of the alkali halides when irradiated for short times at room temperature (RT) show only the F absorption band (besides a broad unspecified band attributed to hole

Radiation Damage Products

513

centers) whose peak position and halfwidth depend on the temperature of observation. The F center has associated with it an emission band whose position is usually different from the position of the absorption band. The F band arises from the Is -> 2p transition and in the presence of perturbations the higher state splits into a Pi/ 2 and 2 P 3 / 2 states due to spin-orbit coupling. These two separate transitions are however observed only in the case of cesium halides even in the absence of external perturbations. Since the electron in the F center is a paramagnetic entity it should reveal, in suitable experiments, information about its surroundings. ESR and ENDOR have been valuable aids in seeking such information. In most cases the F center electron gives an inhomogeneously broadened, angular independent, single paramagnetic resonance line with a g value slightly less than the free spin value. The slight g shift can be explained as arising from a spin-orbit interaction between the electron spin moment and the magnetic field due to its orbital motion. The inhomogeneous broadening is due to the hyperfine coupling with the neighbouring nuclei. If this F center electron is placed inside a shell of six identical nuclei (each nucleus with a characteristic spin) as it is in the case of NaCl-type crystal one should expect to see, if resolved, several different resonance lines. The spacing between the lines gives information concerning the hyperfine interaction. In the case where there are six ions surrounding the F center (with each ion having a nuclear spin of 3/2) one will obtain a nineteen line ESR pattern because of the hyperfine interaction of the electron of the F center with the surrounding nuclei. In KC1 unresolved ESR Signal is observed while in RbCl and NaF resolved ESR spectrum is observed. The number of hyperfine lines in CsCl are different because of the different coordination number. In cases where the hyperfine lines are not resolved in a simple ESR experiment (as in the case of a KC1 crystal) one resorts to the more powerful ENDOR experiment [23]. From such experiments one can get information about the interaction of the electron of the F center with the nuclei of the more distant neighbour shells. Thus the interaction of the F center electron with the surrounding shells can be studied by ESR in some cases and by ENDOR in many cases. When most alkali halide crystals are heated, after the F centers are formed by radiation damage, the F center electrons are released and these electrons on recombination with suitable hole centers give thermoluminescence peaks at temperatures governed by the trap depth of the F center level. The wavelength of such thermoluminescent emission is dependent on the host material. However, even in the so called highly pure crystals at least two thermoluminescence (TL) peaks which can be associated with the F centers are reported [14-1-5] in several alkali halides. These two TL peaks are ascribed to two kinds of F centers with slightly different trap depths. The temperature at which these TL glow peaks appear is also a function of the heating rate [14]. Such TL peaks have been observed, in irradiated alkali halides. From these experiments the trap depth, the thermal ionisation energy, and the thermal emission wavelength (which may or may not be the same as the optical emission wavelength) can be determined. The intensity of TL peaks and the number of TL peaks are a function of the radiation dose, and the purity of the sample. The two TL peaks observed in irradiated crystals are ascribed to two stages of F center formation, the first because of vacancies which are already existing in the lattice while the second is because of F centers formed by the electrons being trapped in the vacancies which are also products of radiation damage. While the first stage and therefore the first TL peak saturates quickly the second stage and the second TL peak increases slowly. More recently [16] it has been shown that even nominally pure crystals give rise to several TL peaks and this has been ascribed to TL arising from the recombination of the electrons from the same F center and different hole centers. This assignment is based on the fact that the emission wavelength is different for each of the TL peaks. However, other experiments [27] where more than two TL peaks are observed interpret them as coming from F centers in different environments. 38*

514

S. Radhaxbishna and B. V. R. ChoWdabi

These different environments may not give rise to different trap depths but they may arise because of different frequency factors S. The frequency factor S is apparently more sensitive to the surroundings in which the F center is situated. Thus several of the observed glow peaks may be attributed to the F centers in different surroundings. For instance, an F center in the neighbourhood of an impurity ion would give rise to a different TL peak from an F center which is isolated although both of them may have the same activation energy. This suggestion appears relevant in view of the quality of crystals normally obtained. It should also be noted that although the TL emission peaks occur at different temperatures they all have the same emission wavelength. In crystals irradiated at LNT another important product is the F' center which consists of two electrons trapped at a single vacancy. F' center gives a broad optical absorption band on the long wavelength side of the F band. For example in KC1 it is found at 750 nm (at 170 It). Thermoluminescence of crystals irradiated at low temperatures [18—20] also yields considerable information regarding the radiation damage products. Careful F light stimulation experiments on TL in crystals irradiated at 80 K enable the identification and confirmation of a TL peak at 194 K as one arising from the F' center electron. A trap depth of 0.52 eV is deduced for such a trap. Such optical stimulation experiments enables an unambiguous identification of the F' center TL peak as no other hole centers are allowed to be formed during the process of formation of F' centers (only electron excess centers are formed by F light bleaching at LNT of the samples irradiated at RT). Such glow peaks associated with the F' centers are reported at 187 K (trap depth 0.32 eV) and 219 K (trap depth 0.40 eV) in KBr and KC1, respectively. II.2. Effect of impurities on F center formation: All the impurities that have been incorporated in the crystals influence the formation of defect centers in the lattice in one way or the other. Some divalent impurities like Ca, Sr and Ba indirectly influence the number of F centers formed by altering the equilibrium number of vacancies (thus influencing the growth rate) or by being in the neighbourhood of electron excess centers or hole excess centers (thus altering the environment of the host center). This later process perturbs the energy levels and gives rise to new absorptions bands, Z centers if divalent impurities [21] and F^ centers if monovalent impurities [22]. Some other divalent impurities like Cd, Zn, Bi, Pb and Sb participate directly [23] in the process of radiation damage and provide additional traps for the electrons and/or holes produced in the process of radiation damage. In such cases the growth rate is usually found to be suppressed in both the first and second stages. The altered valence states of the impurity can give rise to new absorption bands, induce ESR signals and give rise to luminescence if they are luminescent centers. The presence of monovalent impurities like Na, Li, or K (which constitute isotopic substitution) at one of the equivalent substitutional sites give rise to the FA or the HA centers (after irradiation) depending on whether it is in the neighbourhood of an electron trap center or a hole trap center. Such centers have been studied and their models are unequivocally established by ESR and ENDOR experiments. However, the F^ centers have not been investigated in detail in irradiated crystals probably because of the low concentration of centers. From the available literature regarding the effect of impurities on the growth rate of the F center it can be concluded that if the impurities are electron trapping centers they inhibit the growth rate while they enhance the growth rate if they do not act as electron trapping centers. This also depends on the temperature of irradiation. For instance Pb impurity suppresses the growth of F centers for irradiation at RT while it

515

Radiation Damage ProduGts

enhances for irradiation at LNT. There are however a few controversies in literature regarding this general result. I t has been found [24] that Ca, Sr, Mn and Cd increase the growth rate proportional to the square root of the concentration of the impurity. I t was also found in these experiments that ionic conductivity was decreased in such a way that the decrease in the concentration of the positive ion vacancies was nearly equal to the increase in F center concentration. From these results it was concluded that isolated positive ion vacancies are responsible for the first stage F centers. Care was taken to see that the crystals are free from OH impurity which is a major source of positive ion vacancies. I t has also been shown [5] that some impurities act as traps wavelength 0.5 0.6

"i

ZJÎeV

n

I

KCUF

7 2 /um I r 7"K 7.24eV n

1.0

*tp 3p

iCB

3S

•T5eVt 2P'

•neV

%

CNI ^ 2.5

2.0

1.0 eV

Is

Photonenergy (a)

(b)

Fig. 2. a) Schematic model of the F center and typical absorption and emission bands in a KC1 crystal b) Schematic energy level diagram for F center and excited states

for the mobile interstitials and thus enhance the colorability by impeding the recombination of interstitials and vacancies. Further it was shown that several different impurities of different valence states, and different sizes enhance the growth rate if the irradiation is done at LNT. This enhancement appears to be insensitive to the chemical properties of the impurities. However if the irradiation is done at RT there are conflicting reports about their influence on the growth curves [25]. I t is also known that for zone purified crystals the growth curve lies below a similar curve for crystals which are not zone purified. Thermoluminescence studies in impurity doped crystals can also give considerable information. Impurity doped crystals give rise to new TL peaks which may either be interpreted as peaks due to F centers in the neighbourhood of the impurity or as impurity peaks themselves. In addition to this the two F center peaks are reduced in intensity in some cases but altered practically in all cases. From these studies one can get an idea as to which impurities suppress which stage of the F centers [14, 15, 27]. The impurity peaks will be discussed later. As representative examples of the F centers that are formed on radiation damage Fig. 2 and 3 are given for optical and ESR spectra. In Fig. 2 besides the F-band transitions the other optical transitions which are usually found as accompaniments to F-bands are also shown. These bands are usually weak and unresolved but become perceivable for high F center concentrations. Fig. 3 shows examples of resolved and unresolved F center ESR signals. Fig. 4 shows typical TL spectra in irradiated alkali

516

S. Radhakrishna and B. V. R. Chowdari

KCL i

i

i

i

i

i

i

i

i

i

i i

i

i

i

i

i n

i

i

i

i

i n

i

i

i

i

2.6 2.8 3.0 3.2 6.4 3.6 3.8 4.0 42 kG

2.6 3.0 3.2 '3.4 3.8 40 kS si 2.8 — 3.6 -i 42 i ir i ^ i 11 i 1 i g = 2.00

Fig. 3. Typical E S R s p e c t r a ofFcenters in KCl, RbCl (Ba || [110]), NaF [13a] (T = 300 K . v = 9380 Mc/s), and CsCl [136] ( T = 80 K ) .

40 80 720 760 200 240 Temperature

(°C)

40 80 120 160 200.2U0 Temperature

(°C)

Fig. 4. Thermoluminescenceof K B r crystals irradiated for different lengthsof time. Curves 1 to 5 are for 15 min., 30 min., 2 hrs., 6 hrs., and 18 hrs. respectively. Thermoluminescenceof NaCI crystals. Curves 1 and 2 are for highly pure and B D H analar purity crystals irradiated for 30 mins. Curves ;3, 4 and 5 are for B D H analar purity NaCI crystals, and NaCI crystal containing 30 ppmof Cd and 35 ppmof Ca respectively. Each crystal is irradiated for 5 mins. [15]

517

Radiation Damage Products

halides. Table 1 summarises the peak positions, half widths and other parameters of the F centers. Similar table for ESR results has not been given as it has been covered extensively in another article [13]. Table I Optical absorption and emission parameters of F centers in alkali halides Lattice Absorption peak Half paraposition at width meter LHeT (5 K)

Absorption peak Half position at RT width (304 K)

Emission peak position at T->0K

Half width

A

eV

nm

eV

eV

nm

eV

eV

nm

eV

1

2

3

4

5

6

7

8

9

10

11

LiF LiCl LiBr Lil NaF NaCl NaBr Nal KF KCl KBr KI RbF RbCl RbBr Rbl CsF CsCl CsBr Csl

4.017 5.13 5.49 6.00 4.62 5.628 5.96 6.46 5.33 6.28 6.59 7.05 5.63 6.54 6.85 7.33 6.01 4.11 4.29 4.56

5.081 3.257 2.771 3.419 3.707 2.746 2.346 2.064 2.874 2.297 2.060 1.873 2.410 2.036 1.851 1.706 1.877* 2.170* 1.931 1.641

244 381 448 362 334 452 528 600 432 540 602 662 514 609 670 725 660 571 642 755

0.689 0.377 0.319 0.526 0.374 0.270 0.355 0.257 0.263 0.191 0.160 0.144 0.194 0.151 0.144 0.129 0.129** 0.130** 0.142 0.239

4.934 3.153 2.673 3.358 3.593 2.655 2.262 2.014 2.787 2.197 1.966 1.783 2.337 1.949 1.761 1.628 1.847* 2.076* 1.802 1.637

251 393 464 369 345 467 548 616 445 564 630 695 530 636 705 762 672 598 688 757

0.804 0.166 0.559 1.259 0.468 0.462 0.570 0.486 0.499 0.375 0.428 0.357 0.361 0.339 0.358 0.319 0.212** 0.346** 0.313 0.664

1.665 0.975

745 1,270

0.39 0.337

1.66 1.215 0.916 0.827 1.328 1.090 Ó.87 0.81 1.42 1.255 0.910 0.760

747 1,020 1,355 1,500 935 1,135 1,425 1,530 872 987 1,360 1,630

0.385 0.261 0.215 0.185 0.335 0.237 0.190 0.148 0.10 0.245 0.184 0.140

Crystal

Notes to the Table: Absorption data is taken from: R. K. D A W S O N and D . P O O L E Y , AERE Report R5946 (1968). Emission data is taken from: W. B. F O W L E R , Physics of Color Centers, Acad. Press, New York (1968). * In the case of cesium halides F band has a three-band structure. However the peak position of the middle band is given here. ** The half widths of the observed three bands have been summed up and given here as the half width of F band.

In thermoluminescence experiments on 'pure' crystals for irradiation at RT one does find interesting dose dependence with respect to the two TL peaks discussed earlier. One of them, the TL peak occurring at lower temperatures and ascribed to the initial vacancies capturing the electron, saturates quickly while the second TL peak increases slowly with radiation time. Such results in the case of KC1 are shown in Fig. 5. The second TL peak can be correlated with the linearly increasing second stage of the F centers. That such a correlation is indeed possible has been shown in the case of KC1 crystals where careful simultaneous optical and TL measurements have been performed [14].

518

S. RADHAKRISHNA a n d B . V. R .

CHOWDABI

The width and the position of the F center ESR signal is not effected by the temperature of irradiation, temperature of measurement or the dose of irradiation. The intensity or the area under the F center resonance signal is only a function of the radiation dose. Further when the hyperfine structure is resolved the hyperfine constant is dependent on temperature of measurement. In the special case of LiF it has been demonstrated [26] that the width of F center resonance signal is strongly dependent on the radiation

Fig. 5: Thermoluminescence of KC1 crystals irradiated for different lengths of time [14] Curve

60

1 — 30 min. 4 - 3 hrs. 7 - 1 0 hrs. 10—24 hrs.

2 — 1 hrs. 5 — 5 hrs. 8 - 1 4 hrs.

3— 2 hrs. 6 - 7 hrs. 9 - 1 7 hrs.

100 no 180 220 250 C TEMPERATURE

dose. In this cage the width of the F center resonance signal shows a mono tonic decrease of the half width {AH) with dose from 130 G at 1016 n/cm 2 to 90 G at 1017 n/cm 2 . At doses as high as 1018 n/cm 2 the width is found to be as low as 1.8 G. The narrowing of the F center resonance with increasing neutron dose is accompanied by a gradual shift in the line shape from Gaussian to Lorentzian. In spite of the changes in the envelope shape and width, the hyperfine shape and angular dependence remain unaltered. A slight reduction in the g value is also reported. Such a reduction in the line width has been ascribed to an exchange narrowing mechanism. Fig. 6 shows such line narrowing

L

(c) Fig. 0. ESR spectra of F centers in irradiated LiF crystals. a) 10" n/cm 2 , b) 5-10" n/cm s and c) 5 • 10" n/cin'. For (a) and (b) H is intermediate between [111] and [110] and for (e) H is along [112]. (After R. KAPLAN and P. J. BRAY, Phys. Rev. 129, 1919 (1963)).

in the case of LiF. If the radiation dose is further increased to 1020 n/cm 2 one observes an ESR spectrum characteristic of the conduction electron spin resonance (CESR) due to conduction electron in small particles of the Li metal. Such crystals acquired a characteristic metallic sheen and the ESR signal had a characteristic Lorentzian shape. These

Radiation Damage Products

519

CESR lines do not disappear even if the crystals are heated to 450 °C unlike the exchange narrowed F center resonance line. Although such exchange narrowing and CESR spectra have not been reported in the case of other irradiated alkali halides there appears to be no reason why they cannot be observed. The growth of absorption under the Fband has been investigated in detail \17a, 17b] and several substages have been analysed. II.3. a Center An a center is an isolated negative ion vacancy and is one of the primary products of radiation damage. I t is formed when a halide ion is ejected out from its place leaving behind a vacancy. Such an ejected halide ion takes an interstitial position and is designated as an I center. The a — I pair is therefore a Frenkel pair. A careful study of the optical absorption on the low energy side of the fundamental bands in K I [27] and other alkali halide crystals (28) indicates the presence of some

Fig. 7. Typical absorption spectrum of a K B r crystal irradiated at L H e T [100]

new bands in that region. These bands designated as the a, (3 and y bands have been attributed to the excitons perturbed by the isolated negative ion vacancies, the F center, and the interstitial halide ion, respectively. I t was concluded that the a band was due to a localised exciton produced in the neighbourhood of a vacancy. Fig. 7 shows the optical absorption spectrum of the a t a n d along with the other bands. Table I I gives the peak position and the oscillator strength of the a band in various alkali halides. An important evidence in favour of the model of the a center emerges form thermal bleaching experiments on crystals containing F centers. Such experiments satisfy the reaction ct + F ' ^ 2F. The a centers are produced in very large numbers if irradiation is done at LHeT but are not produced so profusely if the irradiation is done at LNT and are not at all produced if irradiation is done at RT.

520

S . RADHAKBISHNA a n d B . V . R . CHOWDABI

This can interpreted by correlating the thermal stability of a centers "with the Stability of its counterpart (I center). The recombination of a and I centers, which depends on the distance between a and I centers [29—31], has been explained in terms of an interstitialcy mechanism [32], The a centers remain in the form of a centers and do not get converted to F centers because of the electric field created by interstitial halide ions which/ disturb the original excited 2p state and such a disturbance will make the Table I I Results of the optical measurements of the a centers Crystal

NaF NaCl NaBr KCl KBr KI RbBr Rbl CsBr

Temperature of measurement

Peak position

Half width

K

eV

nm

eV

77 77 ~90 77 77 77 ~90 ~90 77

9.428 7.166 6.230 6.945 6.102 5.209 6.048 5.166 6.048

131.5 173 199 178.5 203 ' 238 205 240. 205

0.4 0.3

1.13

—

—

0.2

Oscillator Decay Ref. strength temperature K

1.18 0.93

280 295 —

202 133 240 225 220 —

a a, M a,

[28] [28]

[1, 2, [27]

27]

[5]

[5]

m

electron prefer the conduction band to the Is ground state [33]. The majority of the a centers are bleached by warming the crystals to 21 K in K B r . However some a centers are present even above 21 K and more information about this is obtained from experiments on chlorine doped K B r crystals [34]. I t has been found that a mixed Frenkel pair involving a bromine vacancy and an interstitial chlorine are formed. Fig. 8 shows the growth curve of the a centers in both pure

X- RA1

—o


2jig) occurs in the IR region. In some cases the I R band is found to have two components and this has been associated with the splitting of the 2jig state into 2 jr jl)a and 2 n g m states due to spin orbit inter-

522

S. RADHAKRISHNA a n d B . V . R . CHOWDARI

actions. Table I I I gives details regarding the optical bands associated with Vk centers in various alkali halides. The Vk centers take preferential orientations b y optical bleaching predominantly b y reorienting through 60° [40, 41]. A change in the orientation Table I I I Optical absorption parameters of the Vk type centers Lattice

TemperaPeak position ture of spacing measurement

Half width

K

A

eV

eV

1

2

3

4

5

6

LiF

77

2.83

77 77 77 4

3.61 3.26 3.97 4.55

KC1

77

4.43

KBr

77

4.65

KI

77

4.96

348 (a) ~ 750 (b) 394 (a) 367 (a) 378 (a) 432 (a) 880 (b) 365 (a) 750 (b) 385 (a) 750 (c) 900 (d) 400 (a) 585 (c) 800 (d) 1,150 365 (a) 4Ò5 (a) 793 (b) 410 (a) 430 (e) 840 (f) 1,000 (g) 382 (e) 760 (n)

1.2

LiCl NaF NaCl Nal

3.563 1.653 3.147 3.380 3.280 2.870 1.409 3.396 1.653 3.220 1.653 1.378 3.099 2.119 1.550 1.078 3.396 3.061 1.559 3.024 2.883 1.476 1.240 3.245 1.631

RbCl Rbl

77 4

4.61 5.16

Csl : Na KC1*

4 11

4.56 4.43

IC1--V, KCl-Pb* 77 BrCl--V t

4.43

nm

1.47 0.66 1.12

Relative intensity of bands

Optical disorientation temperature K

Decay temperature

7

8

9

> 200 1

113

— — —

52 0.81 0.37 0.73 0.26

100 1 445 9.5 1 340

—

0.55 0.36 0.22 0.19 0.76 0.35 0.60 0.23 0.10

173

Ref.

K 10 [39]

(i) (j) 150-160 (i), (k> 58 (1) 208

143

[39,35]

[39]

93

100

[39]

100

125

(i) (1)

120

[36]

234 and 270

[62]

—

56 1

8 1 0.005 94

Notes to the Tables: X refers to a halide ion. * refers to XY~ type Vk center. XY~ type V t center has no center of inversion and hence no V , 'g' classification. 1 and 2 refers to bonding and antibonding orbitals, respectively. X 2 spacing refers to the distance between two nearest neighbour halide ions in the perfect lattice. (a) 2 £ u (c)

*EU

(i) (j) (k) (1)

C. G. R. R.

(e) 2 S l + (g)

• 2Sg • "jigiiz 2+ i72 ->• 2Jt3/2

transition, a transition, a transition, a transition, n

2 (b) 2ZU 7ig 2 2 (d) ZU -*• 7ig3i2 (f) ' 227,+ -»• (h) 2 2, + -»- 2 n,

transition, transition, transition, transition,

n n a n

J . DELBBCQ, T . L. GILBERT, W . HAYES a n d P . H . YUSTER. (unpublished), D . JONES, P h y s . R e v . 160, 539 (1966). B . MURRAY, A E C P r o g r e s s R e p o r t N Y O - 3 8 4 (1969). B . MURRAY a n d F . J . KELLER, P h y s . R e v . 153, 793 (1967).

Radiation Damage Products

523

of the Vk center requires a displacement of one molecule ion in the lattice and the mechanism for disorientation and diffusion is the same. Exceptions to this are I 2 ~'in R b l and N a l [42] and F2~ in LiF [43] for which there is some probability of a 90° jump. ESR study of the Vk center has enabled an unambiguous identification [35, 44, 45, 46]. Fig. 11 shows a typical ESR spectrum of the Vk centers in KC1 crystals in specific » 1 1 na [no] I

3.1

J

I—I

32 1

,

I

1—I

3.3

3Â 1

'

3.5

, t—I—i—i—}

H11 [110]

B-60"

> « « .3,0 IT e-so°

3.0 J

3.7 I

32 l

» »t j,3 lu

3y5 3.fi

8-35.26°

3.3 3A I

I

3.S 3.6 i

i_

Fig. 11. Illustration of the anisotropy of hyperfine interaction in the Vt center in KC1. Frequency 9.3 • 10" c/s. The arrows mark the Cl23s spectra [46]

HIL0ÖAUSS

magnetic field orientations. If the magnetic field is in an arbitrary direction there are six equally probable orientations of the Vk centers and the ESR spectrum will be a superposition of these six spectra. However, if the magnetic field is applied along the (110) direction there are only three nonequivalent directions corresponding to the X 2 _ molecule making 0, 60° or 90° with respect to the magnetic field and hence there are only three sets of spectra. The hole in the Vk center is shared equally by the two halogen ions and interacts strongly with the magnetic moments of the two nuclei resulting in a resolved hyperfine structure consisting of seven lines in KC1 (nuclear spin of CI35 is 3/2) and a three line spectrum in LiF (nuclear spin of F 19 is 1/2). The intensity distribution of the seven line pattern is 1 : 2 : 3 : 4 : 3 : 2 : 1 . This spectrum is further complicated by the presence of chlorine species having the two other possible isotope combinations Cl35—CI37, CI37—CI37. The analysis of the axially symmetric ESR spectrum gives the values of the g shift and anisotropy of the hyperfine interaction. The g shifts arise from the admixure of the excited states with the ground orbital state of the molecular ion. The transition 2su --> 2nu is responsible for the g shift in the ESR spectrum but is forbidden for electric dipole transitions. Table IV gives details regarding information obtained from ESR studies of ~Vk center. The ESR of the \ k center in Csl crystal has recently been studied (36) and it has been shown that the symmetry of the center is along (100) direction. ENDOR of the Vk centers has been studied in few alkali halides [47, 48] and these studies reconfirm the model of the Vk centers first proposed on the basis of

S. Radhakrishna and B. V. R. ChoWdaei

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S . RADHAKRISHNA a n d B . V . R . CHOWDABI

Table V y") for Hyperfine interaction constants (Mc/s) of the ~Vk type centers and orientations (a 0 , different nuclei with respect to the molecule ion principal axes (x, y, z) measured with ENDOR Crystal

Center

Nucleus

4,

Ay

4

1

2

3

4

5

6

7

8

9

LiF«»

vt

A

(Li)

0

34 ± 1

34 ± 1

0

C

(Li)

17 ± 1

0

17 ± 1

D

(F)

0

12 ± 1

12 ± 1

E

(Li)

-5.320 ±0.008 -8.170 ±0.008 -1.820 ±0.010 -2.360 ±0.012 ±2.360

0

(F)

0

39 ± 1

39 ± 1

F

(F)

±1.020 ±0.005 -9.055 ±0.012 -2.450 ±0.006 ±2.170 ±0.012 -1.370 ±0.003 -2.050 ±0.006

0

B

-8.060 ±0.005 -2.840 ±0.012 ±2.800 ±0.010 -3.390 ±0.014 -1.470 ±0.010 -2.090 ±0.006

±13.020 0 ±0.006

0

0

A

-7.15 ±0.04 (F) 0.85 ±0.05 (Na) 1.40 ±0.10 (F) -1.56 ±0.04 (F) -1.10 ±0.03 (F) ' - 1 . 3 6 +0.15

-4.10 ±0.04 -4.57 ±0.05 -1.43 ±0.06 3.05 ±0.06 -1.10 ±0.03 ±4.40 ±0.30

-6.36 ±0.04 -3.74 ±0.02 -1.12 ±0.05 -1.22 ±0.03 ±3.95 ±0.03 -1.22 ±0.30

0

0

0

36 ± 1

36 ± 1

0

19 ± 3

0

19 ± 3

0

16 ± 1

16 ± 1

0

0

0

52 ( ± 8 , - 2 )

60 ( ± 4 , - 5 )

35 ± 4

A

(Na)

0

0

(Li)

0

0

0

B

(F)

38

38

0

B'

(F)

32

32

0

C

(Na)

~ 7.5

~

D

(F)

0

16.3

16.3

D'

(F)

0

10

10

F

(F)

-5.50 ±0.04 -4.06 ±0.02 -3.22 ±0.02 -5.01 ±0.02 -1.10 ±0.10 -1.10 ±0.04 -1.73 ±0.04 ±3.77 ±0.02

0

A'

-3.17 ±0.04 ±1.65 ±0.02 -3.945 ±0.02 -5.35 ±0.02 -1.55 ±0.10 ±2.52 ±0.08 ±3.35 ±0.08 -1.06 ±0.04

0

2.5

2.5

NaF«»

v*

B C D F G NaF ( c )

V M (Li)

(Na)

-6.15 ±0.04 -6.04 ±0.02 ±1.34 ±0.06 -0.22 ±0.06 ±2.20 ±0.50 -1.36 ±0.02 -2.77 ±0.02 -1.06 ±0.04

ß°

~

15.0

15.0

Notes to the.Table: A and A', B and B', C and C , D and D', E and E', G and G' are all equal when we consider V/( centers. For details of the position of the various nuclei see Fig. 20. (a)-reference [47]. (b)-reference [48]. (c)-reference [80].

527

Radiation Damage Products

ESR results. These experiments further yield information regarding the wavefunction of the unpaired spin. The hyperfine constants for a number of sets of non equivalent nuclei have been determined for Vk centers in several lattices and results are tabulated in Table Y. The role played by the presence of electron trapping impurities (ex: T1+, Pb+, Ag+) in the crystal during the formatibn of Vk centers is interesting [38, 39~\. I t has been found that by the presence of T1+ the production rate of Vk centers for a given intensity of irradiation .is enhanced by about 103 times. This suggests that the formation of Vk centers does not involve vacancy production. Tl + ions act as efficient electron traps and thus prevent the recombination of electrons and holes produced by irradiation whereas in a pure crystal recombination prevents the rapid formation of Vk centers. I t is relevant to note here that the ESR spectrum of the Vk centers formed in crystals which contain electron trapping impurities is the same as that obtained when the crystal is pure except that the intensity is much more. This result shows that the electron trapping impurities do not form a part of the Vk centers but only indirectly aid in the production of Vk centers. The recombination luminescence of Vk centers in various alkali halide crystals has been observed [49—51] and the peak position, half width of the emission bands at 4 K in various crystals are tabulated in Table VI.' The origin for these emission bands is the Table VI Properties of recombination emission bands of Yk type centers at 4 K Crystal

NaCl KCl KCl: ICI KBr KI Rbl* Csl

Peak position

Half width

eV

nm

eV

3.36 5.38 2.31 2.64 3.39 2.27 4.42 3.33 4.00 3.12 3.88 3.65 4.26

369 231 536 470 365 546 280 372 310 397 320 340 291

0.67 0.63 0.58

Refe

m [49]

[63] 0.40 0.41 0.37

m [49] [52] [51]

* Observed at 80 K

recombination of electron and the self-trapped hole upon X-ray excitation. As can be seen from the Table VI, two emission bands have been observed in all cases other than KC1. I t has been shown (49) that if Vk centers in X-rayed crystals or KBr of K1 are oriented with polarised light, and the crystals subsequently irradiated with light absorbed in the F band, the low energy emission band appears partially polarized with the electric vector of the emitted radiation perpendicular to the axis of the Vk center. A similar result occurs [50~\ for the case of 4.15 eV band also, showing that the emission is indeed associated with the presence of Vk centers. Upon excitation of K I crystals with UV light (7.7 eV) similar emission is observed [52] suggesting that the origin of 39

Zeitschrift „Fortschiitte der Physik", Heft 9

S. Radhakbishna and B. V. R. ChoWdaki

528

emission is same whether X-rays or UV excitation is used. I t has been suggested [49] that the electronic levels possible in the field of a self trapped hole may be considered as the excited states of a halide ion molecule (X2—). Based on this suggestion the low energy and high energy emission bands in K1 may be considered as being due to jiu -> 1Eg+ 1 and 1eu H g + transitions, respectively. These studies have given valuable information regarding the energy transfer process in the crystals. When a Vk center is in an excited state with an additional electron nearby a self trapped exciton is said to be formed and characteristic absorption and emission bands are found to be associated with such an entity [53—55\. Table V I I gives details regarding emission and absorption bands associated with a self trapped exciton (Vk + e). The life time of the self trapped exciton is quite large and hence its ESR could be studied in its triplet state. These studies confirm that the hole retains the characteristics of X2~ molecule ion localised on two adjacent halide sites. Table V I I I gives parameters obtained Table VII Emission and absorption bands of the self trapped exciton Crystal

Position of emission bands* Ekm

NaCl KCl RbCl NaBr KBr RbBr Nal KI Rbl CsBr** * ** *** Elm

Position of absorption bands*** El

eV

nm

eV

nm

eV

nm

5.6

221

3.38

366

7.96 7.76 7.51 6.68 6.77 6.60 5.56 5.80 5.77

155 159 165 186 183 187 223 214 215

4.42 4.20

280 295

2.27 2.10

546 590

4.15 3.95

299 314

3.34 3.30 3.48

371 375 356

eV

nm

eV

nm

eV

nm

1.87 1.72

662 720

2.03 2.12 1.90

610 585 652

3.7 3.7

335 335

1.58 1.48

785 837

1.77 1.65

700 750

3.30 3.35

375 370

All the data is taken from reference [53] except for CsBr. Taken from reference [56], All the data is taken from reference [54]. and Ekm are the a transitions while Et is the n transition.

Table V I I I Spin-Hamiltonian parameters of the self trapped exciton* Crystal

D (Gauss)

KBr

2,650 ± 10

CsBr

6,240 ± 100

E (Gauss)

- 4 9 0 ± 15 0

* Data is taken from reference [56].

gz

gx

gy

[110]

[110]

[001]

1.99 ±0.01 1.98 ±0.02

2.08 ±0.02 2.05 ±0.05

2.07 ±0.02

b (Gauss) aE (Gauss)

211 ± 10 129 ± 10 130 ± 30 280 ± 10

Radiation Damage Products

529

from ESR studies on such centers. A direct correlation has been obtained between the ESR spectra and the emission bands in KBr and CsBr crystals. The Vk centers in Csl have been studied [36] by thermoluminescence and two peaks at 60 and 90 K are observed. From these studies it has been concluded that the 60 K peak corresponds to a linear displacement (0° reorientation) of the Vk center while the 90 K TL peak corresponds to a 90° reorientation of the Vk center. II.5. VF Center When a Vk center in the lattice interacts with other lattice defects it is given a special nomenclature. According to this nomenclature it is called a Vf center, if a Vk center is in the immediate vicinity of a positive ion vacancy [57]. VF center can therefore be considered as a hole trapped at a positive ion vacancy and thus as the 'antimorph' of an

BrCr

F center. However, the hole is localised on two neighbouring halide ions adjacent to the alkali vacancy. Because of the vicinity of the vacancy the molecular bond of the X2~ ion is bent and the resulting center has the symmetry of an isoscelos triangle lying in the (001) plane with the vacancy along (110) axis as shown in Fig. 12. All the information concerning the VF center has been obtained from ESR experiments only. The Vf centers have been observed only in the case of LiF, KC1, and KBr doped with K I and Pb 2+ . They have also been observed in NaCl [-57—-59]. The internuclear distance in the F2~ molecular ion is found to be smaller for the Vf center than for the self trapped hole. The axis of the g-tensor coincides with the orthorhombic axis of the crystal (110, 110, 001). The VF centers are found to have greater thermal stability than the Vk centers as they could be produced above the annealing temperature of the Vk centers. The g and A values of the VF centers observed in different crystals are given in Table IV. 39*

530

S . RADHAKRISHNA a n d B . V . R . CHOWDABI

II.6. VkA Center If the Vk center is associated with an alkali impurity it has been designated as a VkA center. Such centers have been observed in N a F : Li system and studied by ESR and ENDOR techniques [60]. When NaF: Li crystals are exposed to X-rays at LNT, Vk centers are produced. If these crystals are warmed to 150 K where the Vk centers migrate, the VkA centers are produced because of the association of the mobile self trapped hole and the alkali impurity. The conversion of the Vk centers to VkA centers has been monitored by the use of ESR technique. The association of the Vk center with a Li impurity produces changes in the ESR spectrum which are similar to those observed for Vp centers in LiF. The total ESR spectrum observed is the superposition of the spectra produced by the six different orientations of the VkA centers corresponding to the six (110) lattice directions. The F2~ molecular ion in a lattice generally produces four ESR lines corresponding to the four independent spin states of the two molecular flourine nuclei each of which have a spin 1/2. When the two fluor ines in an F 2 - entity are equivalent only three lines are observed because two of the four lines cannot be separated. If the F2~ molecule is bent then all the four lines can be observed as in the case of V P and VkA centers. The bond angle in the case of the VkA center is 0.6° which is substantially less than that of the VF center 4°). Fig. 13 shows the observed ESR spectrum of the VkA centers in NaF:Li and Fig. 14 shows the orientation of the principal axis of the V ^ center as obtained from the analysis of the ESR spectrum. The principal values of the g tensor and A tensor for VkA center in N a F : Li is given in Table IV. ENDOR studies have provided definite confirmatory models for these centers. Hyperfine contribution from the various nuclei has been observed and given in Table V. ESR Spectrum fl, (60°) H0II

ESR Spectrum

Jr

of Lithium VHA center in NaF o fan°\ R (60'' [mi

of Lithium

VHA center in NaF R2(l'5°),R3(90o),Rlt(90o)

W R

H„ll[100]

„ . c o ,

\

WM

* Quartz

I

Une

Fig. 13. ESR spectrum of Yua centers in NaF. The top recorded trace is for H parallel to [110] and the bottom trace is tor II parallel to [100] [60]

II.7. 1 7 - type Vk Centers Recently Vk centers of the form ICb, BrCl - and IBr~ in KC1 have been studied [58,61,62]. In order to produce these centers, crystals of KC1 doped with foreign halogen ion and Pb 2 + have been irradiated with X-rays or y-rays at LNT and then subsequently annealed

531

Radiation Damage Products

to about 210 K. This procedure introduces Vk centers at L N T a n d bleaches them as the temperature is raised above its decay temperature. The bleaching of Vk centers results in the enhancement of BrCl - or IC1- or IBr~ if the crystal is doped with both B r - and I - . The purpose of the Pb 2 + impurity is to enhance the concentration of V t centers and subsequently XY~ type Vk centers. Thermal treatment causes the Cl2~ ions to migrate through the crystal and when they encounter foreign halogen ions of lower electronegativity, XY~ centers such as BrCl - or IC1~ are formed. These centers have (110) symmetry and they occupy two lattice sites. Just as Vk centers, these XY~ type Vk centers also have optical bands associated with them both in UV and I R regions. The

'—[001]

• F O Na


• 2 n g transition, n (c) Charge transfer transition (d) 2S2 -> 2Zl transition, a 2 (e) -»• 7i9l/2; a transition and (f) 2SU -> 27tgli2 transition, n. (R) J. D. KONITZER and H . N. H E R S H , J. Phys. Chem. Solids 27, 771 (1966).

and this disturbance has been followed by optical dichroism and ESR measurements, I t has been suggested [70] that a change in reorientation can take place by rotation, without translation of the axis, of the strongly bonded two central nuclei. The central nuclei remain the same while the outer nuclei change with each orientation. Such preferential orientation is not possible above 10.9 + 0.3 K in the case of KC1. This temperature has been termed as disorientation temperature. Although the H center disorients at 10.9 K it's decay temperature is 42 K in KC1. This decay is due to the thermal instability of the H center. Thus far we have been dealing with the formation of the H center during the process of irradiation only at low temperatures. There are experiments in which the H centers

Radiation Damage Products

535

are formed by other methods [7Ì]. If the crystals are irradiated at LNT Vk centers and F centers are formed. By F light bleaching of such crystals at LHeT, the electrons released from the F centers recombine with the Vk centers to form a self trapped exciton (Vk centers + electron). Such a self trapped exciton decays nonradiatively to form the Frenkel pairs (F and H centers). This kind of production of H centers supports the excitonic mechanism for the formation of primary defects. I

1

1

0=0°

1

1

1

1

Fig. 18. The ESR spectrum of H center at 4.2 K with magnetic field along [Oil] axis. The principal lines of the 0° and 60° spectra are indicated

Another method of producing H centers is by the photo-destruction of U 2 centers (interstitial hydrogen atom) [72]. When KC1 crystals contaning U 2 centers are exposed to U 2 light (5.27 eV) the U 2 centers are destroyed and U (substitutional hydride ion) and H centers are produced. This can be expressed as H/> + ci s - + Cl s -

H r + Cl£s

Formation of H centers by this method is established by the observation of the characteristic ESR spectrum of H centers. I t has also been shown that excitation in the exciton band region in KBr and K I crystals at 5 K gives rise to both the a — I, and F — H Frenkel pairs by exciton annihilation [73]. 11.10. Li 2 - Center Electron or neutron irradiation of LiF crystals at 77 K produces an optical band at 5430 A and associated with it an emission band at 9000 A [74]. The center responsible for this optical bands has a (110) orientation and is described as an interstitial lithium atom (Li2+). This center has also been described as antimorph of the H center. 11.11. H x Center When a H center in an alkali halide crystal is stabilised in the neighbourhood of a foreign ion, a center designated as HA center is formed [75, 76]. The essential difference between the structures of the H and HA centers arises from the presence of such an impurity which disturbs the originally collinear X43~ ion. In this process the four nuclei which formed two sets of equivalent nuclei now form four inequivalent nuclei. The degree of distortion from the H center axis is determined by the size of the alkali impurity. A schematic model of the HA center is shown in Fig. 12. The optical absorption bands observed [77—79] in the case of H^ centers in different alkali halides is summarised in Table IX. A typical optical absorption band observed in the case of H A center in KC1

536

S . R A D H A K R I S H N A a n d B . V . R . CHOWDAEI

crystals is shown in Fig. 19. The intensity of the optical band has been found to be a function of the concentration of the monovalent impurity. As can be seen from the Table I X the energy of the transitions, expecially the red ones, decreases on going from H to H.a (Na+) to H^ (Li+) center. Such a behaviour may be interpreted on the basis of a slight increase in the Cl2~ internuclear distance as one goes from H to H x (Na+) to H^ (Li+). It may also be observed from the Table I X that it has been possible to form H x centers in KC1 and KBr only when the substituting ion is smaller in size than the host ion. An exception to this is LiF:Na+ system. In this case the Na + impurity is situated at the nearest neighbour site in a direction perpendicular to the X2~ molecular axis.

0 300

WO

500

WAVELENGTH

600

700

(nm)

Fig. 19. H a band formation in KC1 crystals containing different concentrations of N a + . Irradiation was done at 77 K w i t h Co' 0 source for four hours. Curve (b) KC1 + 5.8 p p m N a + (a') KC1 + 0.2 p p m of Na+ (b') KC1 + 5.8 p p m N a + and (c') KC1 + 15.2 ppm of N a + . Dashed curves are for results obtained after optical treatment. All measurements were m a d e at 77 K [75]

The disorientation and decay temperatures of the HA centers are also given in Table IX. I t can be seen that the thermal stability of the centers increases substantially as one goes from H to JJA (Na+) to (Li+). In KBr crystals it has been found that the optical band associated with the H x center has a structure consisting of two bands (406 nm and 415 nm). These two bands were originally ascribed to two configurations of the H^ centers [81, 82], However, more recent [S3] experiments with controlled impurity concentration contradicts this observation. One of the two bands (406 nm) goes down while the other band (415 nm) increases in intensity if sodium impurity is doped in KBr and its concentration is increased. This has been interpreted as the 406 nm band being due to H.4 center and the 415 nm band being due to center in the neighbourhood of another Na impurity ion (designated as HAA center). The optical conversion of H x centers to H centers and the production of H A centers by irradiation at LHeT in Na or Li doped KBr suggests that there is an interaction between interstitial and the impurity, during the formation stage, since no thermal motion of interstitials takes place at LHeT. It may be noted that the number of HA centers is much larger than the statistical probability of finding an interstitial next to an impurity sits. Itoh and Saidoh have interpreted their results [75, 84] in terms of lattice volumes. They have found that the interaction between the nascent interstitial and impurity exists within about 150 lattice volumes. They have also concluded that the cross sections of the interaction of the Li+ ion with the H center crowdion is larger than that of Na + in KBr. Their recent experiments (85) on the temperature dependence of the formation of the HA centers enables them to explain the production efficiency of the F center at different temperatures. In general the ESR spectrum of the ti A center is similar to that of the H center in many respects. However, the four nuclei are now inequivalent. Since the HA centers are stable even at LNT the ESR of H A (Na+) and H^ (Li+) in KC1 has been studied at LNT and lower temper at vires. H^ (Li+) centers can be created even at DIT (192 K). At LNT a strong line at g = 2.02 with a width of 100 G has been observed due to motionally averaged H x center spectrum in KCl:Na. The relaxation time of the HA (Na+) center seems to be unusually large especially when it is compared to H A (Li+) centers. On

537

Radiation Damage Products

cooling to 35 K anisotropic ESR spectrum due to TLA (Na+) centers has been observed in KC1. Fig. 20 shows the H A (Na+) center ESR spectrum when the magnetic field is along the [110] direction. In this case also there are seven hyperfine lines but hyperfine spacing is different. In contrast to the H center spectrum (Fig. 18) the seven groups of lines have almost the same intensity. This enables one to conclude that the CI nuclei in the central part are inequivalent. The symmetry of the TLA center in KC1: Na is C lft i.e. it has only a reflection symmetry through (100) plane containing the H 4 center. The geometry of the Ii A (Na+) centers and H A (Li+) in KC1 are different. For the H x

8-5.7° ,ll IUI

KCl; Naci Hlltvoi V,C£Air«

Hp*-"*"

_J

3000

1 1—-J 1 1 1 L 3200 3i00 3S00 H(0auSS) B.3SJ° HCL: MaCl miLnil V,CENTER T-3S°K

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3000 3200 3k00 3500 H(Oauss)

6-39.3°

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KCiNaCL

«mil^ 3000

•[on]

•

F' ION

O Li+ ION

3200 3400 3600 H(6auSS) Fig. 20

ONa*l0N Fig. 21

Fig. 20. Anisotropic H.i center B S E spectra in KC1 + 150 ppm Na + for three special orientations of the magnetic field B observed a 35 K. The second derivative of absorption is presented and the spectra were obtained after optically bleaching the V* center [75] Fig. 21. Lattice model for the Ha center in LiF [100~[

(Li+) the Cl 2 - internuclear axis makes an angle of 26° with the [001] direction whereas for H^ (Na+) the internuclear axis makes an angle of 5.7° with the [110] direction in the (001) plane. The difference in symmetry between the H^ (Li+) ion and H A (Na+) center is attributed to the off center position of the Li+ ion in KC1 along the [111] orientation of the lattice. The reorientation motion of the H A center in KC1: Li+ has also been studied [S6]. Recently the ESR of H x (Li+) center has been studied in electron bombarded N a F [57] and it is found that these centers are stable up to 215 K although the reorientation of the defect occurs at about 20 K. The ENDOR of H A centers in Na doped LiF has been studied [80, 88] and the results are summarised in Table XI. Fig. 21 will be useful in understanding the data tabulated in Table XI. 11.12. Haa and

Centers

When two alkali impurity ions occupy the nearest neighbour sites on one side of a X 4 3 molecule (H center) the configuration has been designated as HAA center. The ESR spectrum of such a center has been observed [89, 90] in alkali halides containing a heavy

538

S. Radhakrishna and B. V. R. ChoWdabi

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IO -H 00 00

io co I s

&

Ë.S. • -1 H

[

io o

TM ^H •~~ H ~-"- < -tí > 0 -tí-tí o ö o ö o ö ^ - H O - +-H-H-tíco -H« œ -h-h-h-H-H+IT =! t>Ot)M>01Cl¡ÍÚDlOO l> IO lo IOINOOOSOCOOT-ICOO CO có CO I> H CD IO O io 00 os "»""I CÓ co o œ o O i-H IIII M II IIII IIII IIII ilII Mm CS « gi, O W fr g fr, O fr PQ

«Çj

IN co io CO IN o o o oo o OS o o o o o o O OS o o o « o ö T-Í ö e center has a symmetry higher than the H 4 symmetry. The axis of X2~~ is deviated from the (110) direction by 11.5° in KClrNa. Both the H A A and the H A A ' centers have pyramidal motions. Fig. 12 shows the schematic model of the HAA center. Table X gives the details concerning the g and A values of these centers as obtained from ESR spectra. 11.13. 1 7 - Type Interstitial Centers Just as the 'monovalent alkali ions stabilise the mobile interstitial atom, the foreign halogen ions can also stabilise interstitial atoms. Such studies have been first made in KC1: F~ [91] and later in a large number of alkali halides doped with flourine ion [92—94]. X- or F-irradiation at L N T produces FC1~, FBr - , and FI~ centers which have (111) orientation. The production of these centers is not influenced by the addition of electron trapping impurities (Pb + + ) in contrast to the enhancement of the growth of Vk centers in alkali halides doped with electron trapping impurities. The (111) oriented A T centers involve an interstitial atom, as the XY~ molecule is located at one lattice site as shown in Fig. 12. The XT~ centers are very different from the H centers. While the I I center exhibits an interaction of the two cental nuclei with the two outer nuclei no such interaction is found in the 1 7 " centers. The XY~ centers have thus far been formed

HCL'KF-KBr HII[ri1J FBr~

2000

2500 3000 3500

4000 4-500

d=3S"16'

2000

2500 3000 3500

4000 4500

8~54°44'

"TTH—fi H

J 2000 2500 3000

KCLKFHBr HII [100] r -

3500 4000

FBr~

W

V(GAUSS)

Fig. 22. The FBr~ E S R spectra in KC1 at 77 K in F " and Br~ doped KC1 for three different orientations of the magnetic field [92]

541

Radiation Damage Products

only in flourine doped crystals. A typical ESR spectrum of the FBr~ in KC1: F : Br crystals is shown in Fig. 22. The symmetry of the FBr~ molecular entity is C3v. The degeneracy of the orbitals is not lifted when the symmetry is C3v and hence an axialLy symmetric spin resonance spectrum is observed. The various XY~ interstitial centers studied so far and their g and A values are tabulated in Table X. The thermal stability of (111) oriented interstitial centers increases as one goes from FC1~ to Fl _ through F B r - , as can be seen from Table X. The production mechanism of these X Y~ centers is not well understood although several mechanisms are postulated. The preliminary optical absorption data [92] in regard to these centers shows t h a t FC1 - , FBr~, FI~ centers have 2 bands peaking' at 300 nm, 294 nm and 275 nm respectively in KC1, due to the 2e2 £i transition. 11.14. H x Type XY~ Centers When an alkali halide crystal containing both a foreign alkali impurity and a halogen impurity (KC1:Li:Br) is subjected to irradiation at LNT an XY~ center with a foreign alkali ion in the neighbourhood is formed. Two such centers identified [95] thus far are III I

2300

2800

9 = 25° HA(U*)- TYPE BrCL'IN KCl< (li*,Br~) I ÏÏIl[lD0] T'TIK

3300

3800

8 = 291°

H (6)

i—M I I

ha Ui*)-TYPE Brer m Hll[m] T= 77 K

23ÖÖ

2800 '

3300 8=31,6°

3800

m--(u\Br)

H (6)

r m

HA(Li*)-TYPE BrCL'IN HCL-(Li* Br' HII[110] T' 77«

2300

5800

3300

3800

life)

r i g . 23. H,1 (Li + ) type BrCl- ESK spectra a t 77 K for three different orientations of H; Only the lines corresponding to (Br ! 1 Cl s 5 )- which could be identified have been indicated [95]

B a (Li+) type BrCl" center and B A (Li+) IC1~ center. The concentration of Li + impurity in such crystals is large. The thermal annealing of H x (Li+) produces HA (Li+) type BrCl and (Li+) type IC1 - centers in appropriately doped crystals. Fig. 23 shows a typical ESR spectrum of the li A (Li+) type BrCl - for three special orientations of the magnetic field. The analysis suggest that the BrCl - internuclear axis lies in a (110) plane and it makes an angle of 25° with (100) direction and the molecular axis is bent by about 8° towards Li+ in KC1. The center is formed when a mobile interstitial chlorine atom is stabilised by a pair of substitutional Li + and Br~ impurity ions which are nearest neigh-

542

S . RADHAKBISHNA a n d B . V . R . CHOWDAKI

bours of one another. Both the KA (Li+) BrCl~ and H /t (Li+) IC1- centers possess a thermally activated pyramidal motion of 0 3 „ symmetry around (111). Table X gives the g and A values of these centers in KC1. Table XI Hyperfine interaction constants (Mc/s) of the H type centers and the principal axis orientations (a0, /S°, y°) measured with respect to the molecule ion principal axes (x, y, z) measured with ENDOR Crystal Center Nucleus Ax

Ay

LiF

—4.34 ±0.03 -4.87 ±0.04 -6.05 ±0.04

H A (Na)A(Li) B(Li) B'(Na) E(Li) E'(Li) K(Li) C(F) D(F) D'(F) T(F)

-3.93 ±0.04 2.48 ±0.04 -3.05 ±0.10 1.00 ±0.03 0.83 ±0.03 -0.33 ±0.12 -4.33 ±0.04 -6.81 ±0.90 -4.67 0.90 0.55 ±0.08

Ref.

-0.31 ±0.12 1.06 ±0.11 2.18 ±0.60 2.38 ±0.60 0.54 ±0.08

11.15.

-3.0 ±0.02 -3.29 ±0.02 -5.39 ±0.02 -1.62 ±0.03 -1.45 ±0.03 1.15 ±0.02 —3.07 ±0.02 -5.10 ±0.10 -3.92 ±0.10 4.68 ±0.02

~1

23.5 ± 1.0 23.5 ± 1.0

0

0

0

0

0

0

46 ± 2

0

46 ± 2

46 ± 2

0

46 ± 2

~0

28 ± 1

28 ± 1

~1

0

[SO, 5S]

52 ± 2

53 ± 2

8 ± 2

52 ± 2

54 ± 2

15 ± 2

0

0

0

H n Center

In LiF crystals, heavily irradiated, at LNT a different kind of H center is formed. This center, designated as the H N 1 center, is an interstitial diatomic hole center which occupies a single lattice site (just like the H center) but has assymmetric hyperfine interactions with the two fluorine nuclei. The center is stable up to 450 K but the ESR spectrum can be studied only up to 220 K because of line broadening problems [96]. 11.16. I Center X-irradiation of the alkali halides at low temperatures forms the a-centers as a result of the ejection of the halide ion from its regular lattice site. Such an ejected entity stabilises as a halide ion in an interstitial position and is termed as the I center. Such a center [29, 31, 34, 97—100] occupies a body centered position in an NaCl type crystal (local Td symmetry) and face centered position in CsCl type crystal (local Giv symmetry) [10,

6].

Optical absorption studies of the I center have been made in KC1, KBr, and CsBr crystals only and the results of these measurements are given in Table X I I . Although the nature of the optical transition responsible for the I band is not clear, it seems to arise from a transition from the ground state of the p 5 configuration to the nearest excited state [99].

543

Radiation Damage Products

The effect of impurities on the thermal stability of the I centers has been studied [98, 99] with Li + and Na+ impurity. I t is found that if the crystals are irradiated at LHeT there is a relatively weak band at 222 nm in addition to the band at 230 nrn in Li + doped samples. On warming such a crystal to a temperature above 21 K when the isolated interstitial ions (I centers) become unstable, the 222 nm band increases and the 230 nm band practically disappears. The 222 nm band is therefore said to arise from I Table X I I Results of the optical absorption measurement of I type centers Crystal

Center

KCl KBr

I I

CsBr KBr

I IA

(Na)

KBr

IA

(Li)

Peak position

Half width

Decay temperature

Ref.

eV

nm

eV

K

6.358 5.375 5.687 5.253 5.375 5.766 5.560 5.793

195 231 218 236 231 215 223 214

0.30 0.30 o.ho 0.30 0.42 0.30 0.56

18, 28.5 11, 17, 19, 21

[99,

~18 ~140

[10] [97-99]

~195

[97-9Ù]

29]

—

centers stabilised in the neighbourhood of a Li + impurity and such centers have a one to one correspondence with the a centers. Similar results have been observed in Na+ doped KBr crystals where the interstitial halide ion is stabilised in the neighbourhood of Na+. The presence of divalent impurities like Mn2+, Ca2+ and Sr 2+ also provide additional sites for the stabilisation of the interstitial ions. Such an interstitial ion in the neighbourhood of a Mn 2+ ion is amenable to studies by the powerful ESR technique. In particular, very large crystalline fields are characteristic of such centers associated with Mn 2+ ions [«]. 11.17. Interstitial Aggregate Centers J u s t as the F aggregate centers are produced by long X-irradiation, or F light optical bleaching, or thermal bleaching, there is evidence [4—6] for the existance of similar centers arising from aggregates of interstitials. I t has been shown that prolonged X-irradiation at LHeT or optical or thermal bleaching of H band produces an optical band at 248 nm, 278 nm and 320 nm in KC1, KBr and CsBr, respectively [101, 78, 10]. These bands are designated as H' bands in these crystals. There exist a linear relation between H' bands and the square of the H band indicating that the center responsible for H' band is composed of two H centers. The detailed experiments on the H' band gave indication that the shape of the H' band changes with irradiation time and annealing temperature indicating that it is composed of several bands [102]. At LHeT, two H centers can interact in several configurations since at this temperature most of the configurations are stable and hence a composite H' band is observed. As the temperature is raised some of the di-H center configurations become unstable and as one approaches the temperature around 150 K probably the most stable configuration results. Indepedent experiments on X-irradiation at about DIT shows that a prominent band at 240 nm, 276 nm and 276 mn is produced in KC1, KBr and CsBr, respectively, and these are designated as V4 bands [10, 103—105]. These bands grow in proportion to F 40

Zeitschrift ,,1'ortschiitte der Physik", Heft 9

544

S . R A D H A X R I S H N A a n d B . V . R . CHOWDARI

band indicating that the center responsible for them (V4 center) in various crystals is complimentary to F center. Further, it was also shown that the photoexcitation of these bands at LHeT produces H bands and the thermal bleaching of such H bands produces V4 bands indicating that the interstitial halogen atoms are involved in the formation of V4 centers. It has been suggested [104] that the V4 center is also a di-H center. Therefore it may be concluded that V4 center is one of the many configurations of the di-H centers and probably the most stable configuration. I t may be noted here that both in KC1 and KBr crystals the optical band positions of H' and V4 centers are very close to each other whereas in CsBr they differ quite a bit indicating that the structure of the host crystal plays a considerable role in deterimining the optical transitions of the centers. However, recent experiments [106] have shown that the 270 nm band in KBr (V4 band) is a composite band. I t is necessary to see whether this composite nature brings in the apparent differences in the behaviour of KC1 and KBr on one side and CsBr on the other side as far as V4 and H' centers are concerned. Dichroic studies of V4 centers indicate (100) symmetry in KC1, KBr and CsBr crystals whereas H' centers does not show any dichroism. Some theoretical calculations [107] have also been done in order to understand the formation and structure of di-H centers. Considerable effort has gone into understanding the nature of the centers (V2 and V3) responsible for the broad UV band 215 nm in KC1) obtained on irradiating pure alkali halide crystals at about room temperature [108—112]. It has been suggested that the V 3 center is a X3~ molecular ion at a cation vacancy with no specific orientation [107], The V2 center which has an optical absorption band at 230 nm in KC1 (at 200 K) is found to exhibit dichroism which suggests a (100) orientation for the center [108], I t may be noted here that V4 center also has (100) symmetry but it is predominently produced by irradiation at about 100 K only. One conclusion that can be arrived regarding V2 and V 3 centers is that these two centers are also intrinsic and do not involve impurities as their constituents. Table X I I I a give details regarding peak wavelengths for the intrinsic V bands in different alkali halides. It can be seen from the Table X I I I that there is a dependence of the peak position of the V2, V 3 and V4 bands on the halide

Table X H I a Optical absorption parameters of the interstitial aggregate centers Crystal

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