PRODUCTION AND THERMAL ANNEALING OF LATTICE DEFECTS IN KCl

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PRODUCTION AND THERMAL ANNEALING OF

LATTICE DEFECTS IN KCl

A. Behr, H. Peisl, W. Waidelich

To cite this version:

A. Behr, H. Peisl, W. Waidelich. PRODUCTION AND THERMAL ANNEALING OF

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JOURNAL D E PHYSIQUE Colloque C 4, Suppl&nent au no 8-9, tome 28, Aulif-SepfemBre 1967, page C 4- 163

PRODUCTION

AND

THERiiAL

ANNEALWG

OF LATTICE

DEFECTS IN KC1

A. BEHR, H. PBISL and W. WAIDELICI-I

I. Physikalisches lnstitut der Technischen Hochschule, Darmstadt (Germany)

Rhumb. - Recherches sur la production de dtfiiuts dans les halogknuxes alcalins par irradiation X. On effectue sur KC1 des ~nesures associks du changement rnacroscopique de volume et de celui du parametre du rcseau. Des centres colores sont formts dans KC1 colore par addition, de

lacunes dc dtfauts de Schottky, tandis que dans KC1 irradit, ils sont formts de lacunes qui font

partie dc dtifauts de E'renkel. En outrc la production de lacunes. c. A d. de centres cr plus F par les rayons X dans KC1 de diffkrentas origincs, avec diffkrentes densitts de dislocations et differentes impurctds divalentcs, est ttudike la tempdrature de l'hClium liquide. Le comporten~enl pendant le rechaufiement des cent]-es a, F et H et celui d'une bande d'absorption h 195 nm montre que, pendant I'irradiation X h la tempkrature de l'hklium liquide, dcs lacunes et des diifauts interstitiels se formcnt dans KCI, et qu'ils se rccombinent A differents stades du recuit. L'analyse des courbes

de recuit donne les Cnergies d'activation et Ies ordres des riactions. A la suite des experiences d'irradiation et de recuits c o m b i n b aux tempkatures de We et de N Z liquides, on a conclu que le mPme processus primaire doit intervenir dans Ia formation dcs dkfauts A ces deux kmpCratures.

Abstract.

-

Some investigations were performed to obtain more information about the production of defects in alkali halides during X-irradiation. Combined measurements of tIie macros- copic volume change and the change of the Iattice parameter of KC1 were performed. Color centers

in additively colored KC1 are formed from Schottky-defect vacancies, whereas in X-irradiated

KC1 they are formed from vacancies which are parts of Frenkel defects. Further, the production

of vacancies, i. c. a plus F centers by X-rays in KCI of different origins, dislocation densities and

of different divalent impurities is studied at liquid helium temperature. It is concluded that the

generation o f vacancies at liquid helium temperature is an intrinsic process. The annealing beha-

viour of a, Fand Hcenters and the annealing of an absorption band at 195 nm shows, that during X-irradiation at liquid hclium temperature vacancies and interstitials are formed in KC1 which

recombine in distinct annealing stages. An analysis of the annealing curves gives the activation energy and the order of reaction. It is concluded from combined irradiation and annealing experiments a t liquid helium and at liquid nitrogen temperature that the same pri~nary process should

be valid for the defect formation at liquid helium and at liquid nitrogen temperature.

Introduction.

-

Though

KC1 with color centers about the type of disorder, which color centers repre- has been investigated many times, little is known sent in thc crystal Iattice, can be obtained from combi- about the mechanis~n by which they a r e formed ned measurements of the macroscopic volume change during irradiation [l]. There are some indications and the change of lattice parameter. B centers were that during irradiation of alkali halides color centers introduced by additive coloration and X-irradiation are formed as Frenkel defects (see e, g. [2]). Investiga- at room temperature. Lattice paratneter change was tions of the growth of the color center absorption obtained f r o m the shift of the (800)

Cu

Kx X-ray bands especially F band 131 show that a t liquid helium diffration peaks. The volume change was studied b y temperature the creation of F centers should be an measuring the mass-density change using a hydrostatic intrinsic process. The purpose of this paper is to flotation method.

report a b o u t some recent investigations performed The results are given in figure l. Curve A shows to get more informations about the production of the results Tor additively colored KC1 and curve X defects in alkali halides during X-irradiation. shows the results for two different samples of KC1 after subsequent X-irradiation. The dashed lines

I. Volume and lattice parameter change in X-irxa- indicate the expected results for different kinds of

diated and additivcly colored KCI.

-

Informations disorder.

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C 4 - 164 A. BEHR, H. PBISL AND W. WAXDELICH

FIG. 1 .

-

Rclaiivc lattice parameter change An/n and rela-

tive mass-density change

-

113 A p / p in ndditively wlored

(curve A) and X-irradiated (curve X) KCI.

If only Schattky defects are created, the relative rnacroscopic density change is mainly caused by the creation of additional lattice sites and onIy partly

because of latticc pararnclcr change. The concentra-

tion of additional created latticc sites which is equal to the concentration of vacant lattice sites c, can bc obtained from the dcnsity and lattice parameter data :

c, =

-

Aplp

-

3 Anla

.

For pure FrenkcI disorder

-

A p l p = 3 Ao/a

will be expected. Relative density change is due to lattice parameter change and therefore relaxation processes only. The data of figure 1 indicate that color centers in additively culored KC1 are formed from Schottky-defect vacancies, whereas in X-irra- diatcd KC1 they are formed from vacancies which are parts of Frenkel defects.

II. Production of defects at liquid heiium tern- perature. - Though K.Cl is generally the best known of the alkali halides, the properties of its a band are scarcely known. Since tlie production of a centers by irradiation at liquid helium temperature is very important, it secms therefore of some interest to investigate the production rate of all vacancy centers i. e. cr plus F centers.

The crystals were X-irradiated in a liquid helium cryostat by a tungsten anode tube (150 kV, 12 mA).

Filtration of the X-rays through the tube and cryostat

window and

O.l

mm copper foil assured uniform coloration. Optical absorption measurements were made in the same cryostat at liquid helium temperature using a Seya-Namioka type U. V. Spectro- meter.

x KC/

-

Xorfh

KC1 - OoH R ~ d g ? No: Lob A KC1

-

Grown in w r Lob

Fro. 2. - Growth curvcs for R and F centers o f KC1 crystals of differzni pretreatment, origins and purities X-irradiated at liquid t~elium temperature.

Figure 2 shows the resuIts on generation of vacan- cies in KCt of different origins, dislocation densities and divalent impurities, as a function of rhe X-ray dose at liquid helium temperature. Dislocation densi- tics > 108 linesJcm2, determined by counting the etch pits on the surfacc of the crystal were obtained by air quenching from 600 O C and by pressure. Figurc 2 shows that the production rates of cl and F ccnters

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PRODUCIION AKD THERMAL ANNEALING OF-' LA'ITICE DEFECTS I N KC1 C 4 - 165 curves for a and F centers in doped KC1 crystals is

quite different frotn that of the rc purc )) crystals, the

sum of all generated vacancy defects i. c. a plus F centers is always thc same for all sanples. We conclude from the rcsults of figure 2 that the generation of vacancies in KC1 a t liquid helium temperature is a pure intrinsic proccss.

III. Thermal annealing of defects created at

liquid helium temperature.

-

Further informations about the dcfects created by X-rays at liquid helium temperaturc can bc obtained from thermal annealing studies [4-71. Therefore the optical absorption of F, a and H ccn+ciqs and an absorption band a1 195 nIn were studied during annealing. Measurements were made at thc baud maximum during warming up of the crystals at about 1 degjmin. I11 iigu1.c 3 the resuIts

Ra. 3.

-

Thcrnlal annealing of latticc defects in low rcrnpc- rature X-irradiated KC]. X-irradiation limcs : U ; I , ; h ; 135 nm :

8 h ; F : 2,5 h ; 1.1 : 5,5 h. Initial concentratinn of centers no(=)

-

6 X 10' h cin - 3 ; tzo(F)

--

6 X 101 6 cm -'3. Max. tempera-

ture error

+

1 dcg.

are given for four crystaIs cleaved from a single block

of Harshaw KCI. I n each case different irradiation times were used to obtain optimurn coloration.

a and 195 orn band anneal i n the same manner in form of three distinct annealing stagcs. From this

equivalence we conclude that the 195 nm band is very likely related to interstitial chlorine ions. A further hint may be the observed linear relation between the growth of thc U _bandand _{the growth}of the 195 nm band, determined to be S : 1. An analysis of the annealing curves gives the activation cncrgy and the order of reaction. In both cases the stage I ( was found to be a iirst order reaction, while the stage 11, corresponds to a second ordcr process. The activation energies were found to be (25 -t. 3) meV and (30

+

3) meV, respectively. It is not possible to determine both quantities with sufficient accuracy for the small stage III,. The surprising low acti- vation cncrgies for interstitial ion migration can be cxplained by an interstitialcy mechanism

181.

Stage 1, can be ascribed to Lhc annealing of inlersiitial ions which are separated not more than about six lattice sites from their vacancies 191 otherwise all a

centers would become F centcrs. In stagc II, such interst~tials anneal which can not return to cr thcir vacancies by this mechanism. Therefore they must

migrate quasifrec to another vacancy. I n stagc JII, interstitial ions most probabiy migratc really free.

For the F center annealing two distinct anncaling stages wcrc resolved with activation energics (40 -f- 4) meV, first order reaction and (88

+

4) mcV, sccond order reaction, respectively. Since the annealing

or

the H band shows a very similar behaviour, this annealing prowss could be ascribed to the rccombina- tion of F and JI centcrs, j. e. vacallcirs plus cIectrons and interstitial atoms. Stage I,, can be interpreted as a recombination of F and H centers which are separated morc t h a n about six lattice spaccs but are sufficiently close to permit a close pair recombination in a first ordcr reaction. Stage l I A can bc explained to be free migration of interstitia1 atoms. Thc increase of the H band above 37 OK takes place at thc same

temperaturc like stage I l l , in which interstitial ions migrate free. The interstitial ion will elther diKusc to a vacancy, destroying both defects by recombination, or to a

VK

ccnler, with for~nation of an

N

ccntw. 11 is possible too that the H centers can be converted into other hole centers which cause a n absorption in the H band region.

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C 4 - 166 A. BEHR, H. PEISL AND W. WAIDELICH

and annealing cxperitnents at liquid helium and at liquid nitrogen temperature, performed to obtain some information about the defect formation at liquid nitrogen temperature.

X - irradiation time

--

-F

FIO. 4.

-

Vacancy ccnter concenlxation in X-irradiated KC!.

0

irradiated and measured at liquid helium tcrnperature ;

X irradiated and measured at liquid nitrogen temperature ; irradialed at liquid helium temperature measured after

warming u p to liquid nilrogen tcm~erature.

Figure 4 shows llle results on the generation o f U

and F ccnters in t w o Harshatv KCI crystals as a function of the X-ray dose a t liquid helium and at liquid nitrogen temperature, respectively. Circles give the

growth curves for F and a centers at liquid helium temperature ; crosses are from measurements at liquid

nitrogen temperature. It can be seen that the production rate of z centers is greater at liquid helium tem-

peraturc than at Iiquid nitrogen temperature. The P centcr production rate atso i s greater at liquid helium temperature irradiation, however the effect is srnalI compared with that of a centers.

Three crystals wcre irradiated at liquid helium temperature for different times. After measuring the optical absorption at liquid helium temperature the crystals were warmed up to liquid nitrogen ternpera-

ture and the a: and F center concentrations were

determined again. The obtained vaiues were just the same as in crystaIs irradiated with the same X-ray dose at liquid nitrogcn temperature (full circles in figure 4).

From this we conclude that the same primary process should be valid for the defect formation at liquid helium and at liquid nitrogen temperature. During irradiation at liquid nitrogen temperature a remarkabIe amount of' the primary formed defects immediately anneal by recombination of vacancies and interstitials. Mobile jnterstitials are produced simuItaneously with vacancies a t liquid nitrogen temperature. A large fraction of these defects recombine unless interstitials are trapped or stabilized by impurities etc. 'This explains the dependence of impurities, pretrcatmcnt, etc. on the defect formation a t liquid nitrogen temprature.

Summary.

-

Different experiments were performed to get more informations about the defect formation. The principal results of these investigations arc summarized as follows :

l . Color centcrs in additively colored KC1 are formed from Schottky-defect vacancies, .whereas in X-irradiated KC1 thcy are formed from vacancies which are parrs of Frcnkel defects.

2. The generation of vacancies i. e, U plus F centers

in RC1 is a pure intrinsic process at liquid helium temperature.

3. During X-irradiation at liquid heIium temperature vacancies and inkrstitials are formed in KC1 which recombine in distinct annealing stages.

4. Thc same primary process should be valid for the dcfect formation at liquid helium and at liquid nitrogen temperature.

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PRODUCTION AND THERMAL ANNEALING OF LATTICE DEFECTS IN KC1 C 4 - 1 6 7 [4] RUCKHARDT (H.), X. Physik, 1955, 140, 574 ; PJtys.

Rev., 1956,103,873.

[S] ITOH (N.); ROYCE (R. S.

H.)

and Swor.uc~~ows~u (R.), [ l ] See e. g. CRAWFORD [J. H.), Jr. : Tmtures at thelnter- _Phys._Rev.,_1965,_137,_A_1010.

Summer Course On State Physicsl ₁₆₁_GEBIIARDT_{(W.), Phys. Chcm. Solids,}_1962,_23,₁_123.

Ghent, Belgium, 1966. _[7]_GIUI.IAKI_{(G.) PERLYATI}_(A.), _REGUZZO~T_(E.)_and

[?l RALZER {R.), PEISL (H.) and WAIDELICH (W.), PIiys. C ~ i r n ~ m t (G.), Solid Stare Cornm., 1965, 3, 61.

star. sol., 1966, 15, 495. [g] THARMALINGAM (K.), Phys. Cfiem. Solids, 1964, 25, [3] R A ~ C N (H.) and KLLCK (C. C.), Phys. Rev., 1960, 117, 255.