GAMES PEOPLE PLAY WITH INTERSTITIALS (IN ALKALI HALIDES)

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GAMES PEOPLE PLAY WITH INTERSTITIALS (IN

ALKALI HALIDES)

D. Schoemaker

To cite this version:

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Abstract. — A survey is given of the various ways in which interstitial halogen atoms produced by ionising radiation can be trapped in alkali halides. First, the fundamental interstitial halogen atom center, the H-center, is discussed. Then, interstitial centers trapped by, or in the neighbourhood of, various impurities are presented. Particular attention is given to trapping by the following impu-rities : foreign halogen ions, foreign alkali ions or pairs of both. The discussion is limited to a des-cription of the production and the models of these H-type centers and little is said about their sometimes interesting physical properties. A few speculations are offered why certain interstitial centers have not yet been observed. The models of a few paramagnetic diinterstitial centers are also presented.

I. Introduction. — Because the alkali halides are it is likely that a number of parallels exist between the prototypes of simple ionic solids they have played the two types of interstitials.

an important role in defect physics. One can produce A second limitation in this talk is that only the pro-an enormous number of defects of various complexity duction of the centers will be sketched pro-and their by ionising radiation or by other means. The identi- models presented. For details of the analysis and a fication and study of these defects is often interesting s t ud y of the physical properties of these centers, one

in itself, but the knowledge of their structure in the }s referred to the original papers and to the review

simple alkali halides is useful in the determination of p a pe r s by Itoh [1], Kabler [2] and the author [3].

defect structures in other materials having a different

character (semi-conductors, metals), or possessing a „ __ , , . . . . .... . . .

. . . , , 2. The fundamental interstitial halogen atom c e n t e r : more complex crystal structure. ™ . u * T U U • • J J v •

_, , ,,*" • „ c u i-j j . - ii The H-center.—The H-center is produced by

lrra-The defects in alkali halides are traditionally .. ,. „ .. , .., .,, v .. .. . ..

,. . . ^ _, diating an alkali hahde with X- or y-radiation at rather called color centers, or just centers. Ihese centers can . ° x T „ „ . ' . . _ , , .

. . - . . / . . . low temperatures. In KC1, e. g., this must be below be classified in a few large groups among which the .n v • , . • . .. , .

, x ± .i. u i * A , L • * .-.- i 40 K in order to avoid thermal decay,

electron centers, the hole centers and the interstitial

centers are very prominent. T h e electronic and ionic processes that take place

In this talk I will give a survey, which is rather per- d u n ng a n d a f t e r t h e absorption of an x- or y-photon

sonal and therefore incomplete, of interstitial centers. a r e c°mp'ex and not yet fully understood, but it is

What is more, I will limit myself to a subgroup of convenient to accept that interstitial halogen atoms, these, namely, interstitial halogen atom centers. * i > a r e produced. The atomic configuration is,

howe-Indeed, it is well established that ionising radiations ver, not the most stable one for the interstitial. howe-Indeed, (X-, y-rays, electrons) produce interstitial halogen halogen atoms, X0, like to associate with halogen ions,

atoms, X?, and interstitial halogen ions, Xf. The for- X"> t o f o r m X^ diatomic molecule ions whose

mer are paramagnetic and can be studied in great b i n d l n8 energies average aboirt 1 eV for the various

detail by means of ESR spectroscopy, and the struc- halogens. Thus, in KC1 the Ch will associate with a ture of the centers can oftentimes be determined in surrounding substitutional chloride ion, Cls , to form

rather great detail from the ESR results. Knowledge a CI J molecule ion which occupies the same negative about the Xj~ centers is often less detailed although ion site that the CI,- did.

GAMES PEOPLE PLAY WITH INTERSTITIALS

(IN ALKALI HALIDES)

D. SCHOEMAKER

Physics Department, University of Antwerp (U. I. A.) 2610-Wilrijk, Belgium

Résumé. — Les différentes manières dont les atomes interstitiels d'halogènes produits par des

rayonnements ionisants peuvent être piégés dans des halogénures alcalins sont passées en revue. Le centre fondamental d'atome halogène interstitiel, ou centre H, est discuté en premier. Ensuite sont présentés les centres interstitiels capturés par des impuretés diverses ou situés dans leur voisinage. Une attention spéciale est consacrée au piégeage par les impuretés suivantes : ions halogènes étran-gers, ions alcalins étrangers ou paires des deux. La discussion se limite à une description de la pro-duction et aux modèles de ces centres de type H, et peu d'espace est consacré à leurs propriétés physiques parfois intéressantes. On trouvera quelques considérations sur la question « pourquoi certains centres interstitiels n'ont-ils pas encore été observés ? » Les modèles de quelques centres paramagnétiques di-interstitiels sont également présentés.

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C764 D. SCHOEMAKER

The ESR spectrum of the H-center is a beautifully resolved and detailed spectrum and the electronic and geometric structure can be determined from it in great detail [3, 4, 51. The H-center model which pos- sesses D,,, symmetry is presented in figure 1 : the Cl2-, which occupies a single negative ion site, is oriented exactly along a

<

110

>

direction of the crystal. Furthermore, the Cl; exhibits very weak molecular bonds with Cl- ions nos. 3 and 4 along the same

<

110

>,

so that the H-center is sometimes described as being a cl:- molecule ion. However, a description as CI; corresponds closer to the physical reality of the H-center.

( a ) -. .

-

(b)

FIG. l . - Schematic two dimensional model of the

<

110

>

oriented H-center in KC1 in a { 100 } plane. The figure also shows the basic diffusive step, or crowdion motion, of the center which occurs above 40 K. In LiF the H-center is

<

1 1 1

>

oriented.

A study combining ESR and optical absorption measurements [S] has establised that the ground state of the H-center is a

'z:

state originating from a

2 4 4

a, n, n, a, molecular configuration. A strong optical absorption at 335 mm is ascribed to +

orientation and this spatial anisotropy can be detected by sampling the specimen in optical absorption with polarised 335 nm light in two perpendicular directions.

This anisotropic distribution can only be produced and maintained a t very low temperatures, namely temperatures near liquid helium. Only there do the Cl; remain frozen into their orientation. However, they can be thermally excited into other orientations by raising the sample to a sufficiently high temperature. In KCI, the Cl; of the H-center become thermally excited around 1 1 K : at and above this temperature the Cl; jumps am& the six

<

110

>

orientations through 600 jumps while staying a t its lattice site. Warming the specimen above 1 1 K speeds up this reorientation process rapidly, and at 35 K, where the H-center is still thermally stable, the reorientation rate must be of the order of 109 Hz.

The H-center in KC1 decays between 40 K and 45 K. The basic diffusion step is sketched in figure l : the interstitial chlorine no. 1 jumps over the neighbouring site along its

<

110

>

of F- ions, one produces highly mobile H-centers. These diffusing cl: will quickly encounter the substitutional F- impurity ions and form FCl- each of which occupying a single negative ion site [g, 9, 101. The ESR spectra which identify this species also show that it is pre- cisely

<

11 1

>

oriented. This, you recall, is the

easy direction for interstitial centers. A schematic

two dimensional model of this FCI- center in a { 110 )

plane is presented in figure 2 (X must be identified with Cl). The

<

11 1

>

oriented FBr- in KBr : F- has also been observed. Optical measurements [l11 have shown that the FCl- center possesses a strong a polarised transition at about 300 nm and that the disorientation temperature for this center is 40 K. Above this temperature the FCl- jumps among the eight possible

<

111

>

orientations, but stays on its lattice site.

FIG. 2.

-

Schematic two dimensional model in a { 110 } plane of the

<

1 1 l

>

oriented FX- centers in KC1 (X

--

Cl, Br, I).

The FC1- decays thermally at 170 K. The mode of decay is not yet clear. Either an interstitial F: breaks off, leaving a Cl, behind, or it is other way around : a CI: breaks off, moves away and leaves a F;.

An observation possibly favoring the first mecha- nism is that when the KC1 crystal also contains Br- (or I-) impurities besides F-,

<

111

>

oriented FBr- (or FI-) is formed when FCl- decays thermally. This is most readily understood by the trapping of a mobile F: by the substitutional Br, (or I;) impurity. However, one could also argue that there is a prefe- rential association of F- and Br- (or I-) ions on neigh- boring substitutional sites and that an FBr- (or FI-) is formed when a mobile cl: encounters this pair. Further experiments are necessary to settle this question.

3.2 THE BROMINE INTERSTITIAL IN KC1 ? - Sta-

bilization of a mobile CI: by an F; may appeal to the intuition because of the smaller size of the latter. However, we remarked already that size considerations are not necessarily decisive in judging whether an interstitial will be stable or not.

Therefore, the existence of an interstitial bromine atom center in KC1 should not be ruled out. Such a center would be produced in KC1 : Br- by the trapping of a mobile cl: by a Br; possible forming a BrCl- molecule ion occupying a single negative ion site. In our attempts to find the ESR spectrum of such a center we had a serious experimental limitation :

the ESR samples could only be X- or y-irradiated at or above 77 K. Attempts to produce the basic center in KC1 : Br- under these circumstances failed. Only in certain specimens, and after long irradiations, could a weak signal be detected involving a B$. But further studies showed that a Na+ impurity was also involved here (see section 5). Thus the interstitial bromine atom center has not yet been observed in KCl, but we feel that irradiation at suitable temperatures below 77 K and above 40 K (where cl! is mobile) may very well yield this center.

3 . 3 THE INTERSTITIAL-TYPE IOH- CENTER IN KC]. - One never knows. If KC1 : Br- does not work, may be KC1 : I - will yield the interstitial 1: center more readily. An irradiation a t 77 K of KC1 : I - did indeed yield a strong ESR spectrum. Analysis showed that it originated from an IOH- molecule ion occupying a single negative ion site [12]. The model is sketched in figure 3 and it is seen to resemble very much the

<

111

>

oriented FI- center shown in figure 2. The OH- group in IOH- play the role of F- in FI-.

FIG. 3. -Schematic two dimensional model in a { 110 )

plane of the IOH- center in KCI. Note the similarity with figure 2 :

OH plays the role of F.

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0-66 D. SCHOEMAKER

tities of OH-, presumably closely associated with the I-, are introduced in the lattice. The KC1 samples grown this way did indeed show the strong UV optical absorption band characteristics of OH-. A KC1 :

I- specimen grown in an inert atmosphere did not exhibit the OH- absorption, and neither an IOH- center nor another 1: type center could be produced in this crystal by X- or y-radiation at 77 K (see however, section 5).

3 . 4 THE INTERSTITIAL BrOH-, ClOH- AND FOH-

CENTERS. - If, as discussed above, an IOH- can be

produced in KC1 : I- which is very similar to FI-, then maybe an interstitial type BrOH- can be made in KC1 : Br- containing also OH-. The latter impurity must now be intentionally introduced. An investigation of this system did not yield a BrOH- species at 77 K. Presumably the decay temperature of such a center lies below 77 K. However, in unpublished work we have observed an interstitial type BrOH- center in a NaCI : Br- : OH- crystal at 77 K. This streng- thens the belief that it may also be found in KC1 below 77 K.

One can speculate further : possibly interstitial type CIOH- (similar to FCI-) can be produced in KC1 : O H - , or interstitial type FOH- in KC1 : F- : OH-. Experiments on these systems yielded negative results at 77 K. Still the existence of such species below 77 K in KCl, or in other alkali halides, remains a distinct possibility.

3.5 THE INTERSTITIAL IClF-- CENTER IN KCl. - There exists an 1: center, an H-type 1C1- center, in KC1 above 77 K, but it apparently needs the help of an F- ion to be stable [9, 111. It is produced in KC1 :

I- : F - crystals by X- or y irradiation at 77 K , and it is observed together with

<

11 1

>

oriented FCl- and FI-. A warm up to about 130 K converts this IClF-- to FI-. On the other hand optical excitation in the 275 nm FI- band produces ICIF--. The ESR analysis shows that this center is essentially a

<

110

>

oriented ICI- molecule weakly interacting with a single F- ion. Presumably the ICl- occupies a single negative ion site and the F - a neighbouring negative ion site. Whether the configuration is ICIF-- or CIIF-- cannot be determined from the ESR spectra a such.

The analogous BrCIF-- center in KC1 : Br- : F- is not observed at 77 K. It seems likely though that it exists at temperatures below 77 K.

The existence of the (ICId)F- center in KC1 gives confidence that the pure interstitial 1: and ~ r : centers will eventually be found in KC1 (see also section 5).

4. Interstitials involving foreign alkali ions.

-

A second and important class of impurities which can trap interstitials are foreign alkali ions. In this case however, the H-center is stabilised next to the alkali ion which does not become involved to any extent in a

molecular bond with the trapped H-center. This means that these alkali ion associated H-centers, called HA centers, manifests themselves all as Cl; molecule ions, just as the basic H-center, but with geometries and properties dependent upon the particular alkali impurity and the host lattice. In fact, four distinct HA center structures have been observed in various alkali halides and they will now be discussed.

4.1 THE HA(Na+) CENTER IN KC1 : Na'. - Mobile H-centers produced by ionising radiation at 77 K in KC1 : Na+ are trapped next to the substitutional Na' impurity ions and form HA(Na+) centers. The HA(Na+) center is the original V, center [13, 141. The structure derived from the rather complex ESR spectra is shown in figure 4. Trapped next to the N a + is a Cl; whose internuclear axis makes a 5.70 with

<

110

>

in { 100) plane. Other differences with the H-center in figure 1 are : nuclei 1 and 2 are inequivalent, the Cl; molecular bond is bent by about 30, and the weak molecular bonds with chlorines nos. 3 and 4 are different from each other. Clearly the presence of the Na' has lowered the symmetry of the trapped H-center from D,, to C , , .

FIG. 4. - Schematic two dimensional model in a { l00 } plane of the H.\(Na+) center in KC1 : Na+. The two parts of the figure indicate the two positions involved in the Restricted Inter- stitial Motion (R. I. M.). The axis for the Pyramidal Motion of Cqv symmetry around

<

100

>

are also indicated. Note the

similarity with figure 1.

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4 . 2 THE HA(Lif ) CENTER IN KC1 : Li

+.

-

The Lif ion, which is even smaller than N a f , should also stabilise an H-center in its vicinity in KCI. Knowing the structure of H,(Naf), one might expect a similar symmetry for this HA(Lif) but with probably a larger tipping angle. However it turns out that the H,(Lif) structure is quite different from H,(Na+). A standard analysis of the ESR spectrum [l51 (observed after X- irradiation of KC1 : Lif a t 77 K), learns that the Cl; of HA(Lif) lies in a ( 110) plane and that its internu- clear axis makes a 260 angle with

<

100

>.

This standard analysis was performed with the implicit assumption that the Cl; lies statically in an orienta- tion. Very recently the author and Lagendijk 1161 have established that the Cl; does not lie in a { 110 }

plane but that it performs a large angle Librational Motion, L. M., with respect to a ( 110) plane, and that every point along the librational path is equally probable. This new analysis puts the value for the angle with [l001 a t 28.20 rather than 260. One can say that the average Cl; orientation lies in a { 110 )

plane. The HA(Li+) model in KC1 : Lif is presented in figure 5 and figure 6.

Like the H,(Naf), the HA(Li+) possesses a P. M.

and a R. I. M. [17]. The P. M. has C,, symmetry around

<

100

>

: the Cl; jumps around

<

100

>

among the four equivalent average orientations in the { 110) planes through this

<

100

>,

and is therefore qualitatively similar to the P. M. of H,(Naf).

The R. I. M. is depicted in figure 7. The interstitial chlorine no. 1 exchanges moIecular bonds with the three substitutional Cl- ions surrounding it. As a result this motion has C , , symmetry around

<

1 1 1

>.

A combined optical and ESR investigation has established that this R. I. M. is a phonon assisted tunnell- ing motion at liquid helium temperatures. Uniaxial stress experiments at low temperatures [16, 181 have confirmed this but have also yielded an unexpected result : under [l001 stress the Cl; orientations changes continuously. The Cl; describes an octant of a cone around e. g. [OOl], and at high stresses at 4.2 K it approaches the (100) plane perpendicular to the exter- nal stress direction. Raising the temperature above 4.2 K while high uniaxial stress is applied makes the Cl; return to its { l l 0 ) plane.

FIG. 5.

-

Schematic three dimensional model of the H*(Li+) center in KC1 : Li+. The C15 does not lie statically in the ( 110 )

plane but librates along an almost quadrant of a cone with respect to this plane and all positions on this quadrant are equally probably : the center is a freely librating elastic dipole.

FIG. 6. -Schematic two dirhensional model in the { 110 )

plane of the Hn(Li+) center in KC1 : Li+. The Cl; librates with respect to the plane of the figure.

FIG. 7. - Schematic representation of the restricted interstitial motion K. I. M. of the Hn(Li+) center in KC1 : Li+.

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C7-68 D. SCHOEMAKER

The reader is referred to the original papers for a There is however another one, namely the HA(Li+) discussion of the interesting physical properties of center in NaF : Li'. Plant and Mieher who detected the HA(Li+) center in KC1 : Li+. this center [20] established that it does not possess

4 . 3 THE HA(Naf) CENTER I N LiF : Na+

I 1 0

>

F; molecular axis, and reduces the symmetry from D,, to C,,.

Because the HA(Na+) center in LiF is so much like the normal H-center in KC1 and KBr, it was thought, when its ESR spectrum was first discovered by Kan- zig and Woodruff [4] that this HA(Na+) center was the H-center in LiF. This identification was almost inevitable at that time, because only ENDOR measurements are able to determine the presence of Na+. When Chu and Mieher found the H-center in LiF as being a

<

11 1

>

oriented F;, the situation was cleared up.

4 . 4 THE HA(Li+) CENTER IN NaF : Li+. - If the molecular axis had to lie in a crystallographic plane or along a crystallographic direction, then the foregoing HA-center geometries would just about exhaust all possibilities.

FIG. 8.

-

Schematic three dimensional model of the Ha(Li+) center in NaF : Li+. This center possesses no symmetry elements.

any symmetry elements whatsoever. The F;, whose nuclei are inequivalent, makes a

-

340 angle with

<

100

>

and lies in a plane through this direction which makes a

-

140 angle with a { 100 } plane. The model is sketched in figure 8. Unpublished uniaxial stress measurements by Lagendijk and the author have established that this center possesses complex motions, some of which are phonon assisted tunneling motions at liquid helium temperatures 1211.

4.5 THE HA, AND CENTERS. - If one small Li+ or N a + ion can stabilise an H-center to higher temperatures, then pairs of Li+ or Na+ ions should be able to d o the same. Experiments in strongly

(- 1

O/,)

Li+ or Na+ doped KC1 crystals have con-

firmed this [22] and two types of trapped H-centers by pairs of alkali ions have been observed. In the HAA center, the mobile H-center has been stabilised by pairs of LiC or NaC which are nearest neighbours of one another (see Fig. 9). In the HASA center the Cl; is stabilised by a pair of alkali ions which are next nearest neighbours of one another (see Fig. 10 and 11).

FIG. 9. -Schematic model in a { 103 } plane of the HAA center model in KCI. The H-center is included for comparison. The indicated bendings of the various HAA molecular bonds may,

or may not, be correct.

dopcd specimens we sometimes found, after long X-

irradiation at 77 K, a very weak ESR spectrum cha- racteristic of BrCl; - molecule ion in the symmetric (CIBrC1)-- configuration [22, 231. Further study showed that this center clearly involved an interstitial bromine, but its very low concentration, even in samples with a high Br, concentration made it highly doubtful that it represented the basic ~ r ? center.

Sincc N a + is a common impurity in KCI, it was sus- pected that at high Br- coriccntrations there is a probability of having pairs of Na+ and Br- ions next to :ach other in substitutional sites. and that after trapping a mobile C]:, a BrCI; - associated with a Na' impurity would be created. Such a center can be desi- gnated as an HA(Na+)-type BrC1;- center.

Experiments on KC1 samples intentionally doped with Na+ and Br- in various absolute and relative concentrations has confirmed the model of this HA(Na+)-type BrCI; - center which is shown in figure 12. The Br is situated at a symmetric interstitial position next to a Na+ and in between two substitutional chlorines. Because of the presence of the Na', the molecule possesses C,, symmetry and a bending angle of about 15O.

FIG. 12. - Schematic model in a { 100 ) plane of the H,\(Na+)- type BrCli- center in KC1 : Br- : Na+.

In KC1 containing a high concentration of both NaC and Br- there is a reasonable probability of having a Br- associated with two Na' ions. After X-irradiation these crystals showed the ESR spectra of a perfectly linear and symmetric BrC12- - molecule ion associated with two N a + ions, i. e., an HA,(Na+)- type BrCl; -, whose model is presented in figure 13.

q A A ( N a * ) TYPE Er Cl;-

FIG. 13. - Schematic model in a { 100 ) plane of the Hnn(Nai)-

type J?rCI2- centers in KC1 : Br- : Na*.

Because most of the foregoing ESR measurements were done at 77 K , there is a possibility that one is seeing an averaged ESR spectrum, i. e., that the equi- librium position of the Br is ofT the symmetric position shown in figure 9 and 10 toward either one of the two chlorines, and that at 77 K it jumps at a fast rate between these two positions. Such a motion would

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C7-70 D. SCHOEMAKER

be identical to the R. I. M. of the normal HA(Naf)- center in KC1. Experiments at 4.2 K have shown that the Br remains at the symmetric interstitial position. Thus the BrC1;- centers do not appear to possess internal motions.

The existence of these centers suggests that, if and when, the pure ~ r y center is found it may well have this symmetrical (ClBrC1)-- configuration although, of course, a more asymmetric configuration cannot be excluded.

Finally, the HA(NaC)- and HAA(Na+)-type IC1;- centers have also been found in KC1 : Naf : I-.

5 . 2 THE H*(L~+)-TYPE BrCl- A N D ICl- CENTERS IN

KCl.

-

At this stage it is evident that one should try to replace Na+ by Lif and see what happens. An ESR investigation at 77 K of X-irradiated KC1 : Br- : Li+ samples did indeed yield a Li+ trapped ~ r y center 1241, but the center did not manifest itself as a BrCl;

-.

Rather (see Fig. 14) it is a BrCI- molecule ion on a single negative ion site next to a substitutional Li', and the BrCl- axis makes a 260 angle with

<

100

>

in a { 110) plane. An identical HA(Lif) type ICl- center was found in KC1 : I- : L i f .

HA(Li *) Hn(Li.1 - type Br C l -

FIG. 14.

-

Schematic two dimensional model of the H*(Li+)- type BrCI- center in KC1 : Br- : Li+. This model is very similar to the H.4(Li+) center model in KC1 : Li+ as given in figures 5

and 6.

It should be mentioned that these HA(Lif)-type BrCI- and 1CI- centers possess a P. M. around

<

l00

>

just like the HA and HA*, centers.

6 . Paramagnetic diinterstitial centers.

-

6.1 A DIIN-

TERSTITIAL BrCl; - CENTER I N KCl. - In the KC1 :

Br- : Na+ samples in which the HA(Naf)- and H,,(Na+)-type BrC1;- were observed (section 4) it was found, after these centers were annealed out thermally, that another BrCl; - center could be detected [25]. The ESR analysis showed that the internuclear axis of this center made a 20° angle with

<

100

>

in a { 110 ) plane. Because the existence and symme- tries of the HA(Na+)- and HAA(Na+)-type BrC1;- centers are well established, it is not possible to find a reasonable model for this new BrC1;- center based solely on the trapping of a single interstitial in the neighbourhood of a Na' impurity.

One is forced to propose a model involving two trapped interstitials. The specific model is shown in figure 15 : a BrCl; - occupying one negative ion site next to a substitutional N a + impurity. This is very likely produced by the trapping of both an interstitial chlorine atom, cl:, and an interstitial chloride ion, Cli-, by a pair of substitutional and neighbouring N a + and Br- ions.

FIG. 15. - Schematic two dimensional model in a { 110 } plane of the diinterstitial type BrC12- center in KC1 : Br- : Na+. The diinterstitial Fi- center in NaF : Li+ has a similar model.

6 . 2 A DIINTERSTITIAL F; - CENTER IN NaF. - Plant [26] has observed in NaF : Li + crystals a F;-

center whose axis makes a 200 angle with

<

100

>

in a { 110 ) plane. The diinterstitial model he was forced to propose is identical to the diinterstitial BrClt- model presented in figure 12 : the F;- occupies a single negative ion site next to a substitutional Lif impurity ion. Again the center is thought to be formed by the trapping of both a F: and F; next to a substitutional Li+ ion.

Conclusions.

-

It is clear that in alkali halides interstitials are readily produced by X- or y irradia-

tion that they can be stabilised by a large number of suitable impurities.

In the cases that we considered here, i. e., trapping by foreign alkali or halogen ions or both, the structure could in most cases be unambiguously determined from the ESR spectra. This is so because the number of possible trapping geometries around a simple impurity are very limited and a definite choice for the model can usually be made from the knowledge of the symmetry of the ESR spectrum.

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be so straightforward. An example of this situation is afforded by interstitials trapped by divalent positive ions plus their associated positive ion vacancy. An initial ESR investigation by Hayes and Nichols [27], and a large amount of unpublished work by the author on such systems has shown that, e.g. in KCI, these centers are again Cl; type centers possessing a low symmetry configuration.

Because the Cl; is now trapped near a divalent ion and/or a positive ion vacancy many more trapping configurations exist compared to trapping by mono- valent impurities, and consequently the determination of the morphology of these centers is quite difficult.

Evidently, in searching for new centers, interstitial centers in this case, one is playing a game in which one gets more and more confidence in the rules as one goes along. Some of the centers are merely variations on a theme. Others however, have interesting physical properties uniquely their own and their physics make for interesting study.

Although our survey was limited to paramagnetic interstitial halogen atom centers, there are probably many formal parallels with interstitials halogen ion

centers, which are called I centers [l, 21. For instance I, and I,, centers exist although evidently the electronic structure must be quite different.

The knowledge of the structure and properties of these various H-type centers has been, and still is, quite usefull in the study of interstitials in other more complex materials such as, e . g., the alkali earth

halides [28] and the perovskites of the type KMgF, ~ 9 1 .

There are even parallels with interstitials in metals [30]. It is now established that the basic interstitial center (called the self interstitial) in metals is a dum- bell shaped diatomic entity very much like the H- center in alkali halides. Impurity trapped interstitial have also been found. What is further remarkable is that these interstitial centers in metals possess reorientation motions which are very similar to the motions exhibited by the H-type centers in alkali halides with activation energies which are quite comparable.

Acknowledgments. - Support from the Belgian science supporting agencies NFWO and IIKW is gratefully acknowledged.

References

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