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FORMATION DE CENTRES COLORÉS DANS LES HALOGÉNURES ALCALINS-TERREUXRADIOLYSIS
OF ALKALINE EARTH FLUORIDES
W. Hayes
To cite this version:
W. Hayes. FORMATION DE CENTRES COLORÉS DANS LES HALOGÉNURES ALCALINS- TERREUXRADIOLYSIS OF ALKALINE EARTH FLUORIDES. Journal de Physique Colloques, 1973, 34 (C9), pp.C9-499-C9-502. �10.1051/jphyscol:1973983�. �jpa-00215459�
FORMA TION DE CENTRES COLORES
DANS LES HALOGENURES ALCALINS-TERREUX
RADIOLY SIS OF ALKALINE EARTH FLUORIDES
W. HAYES
Clarendon Laboratory, University of Oxford, U. K.
Rhumb. - Les structures des centres colorts dans les fluorures d'alcalino-terreux sont main- tenant suffisamrnent bien connues pour rendre possible les etudes des rnecanisrnes de radiolyse.
Cette courte revue dkcrit les resultats de recentes mesures de I'influence de la temperature sur le rendement de production des centres F dans CaFz et SrFz et comrnente les processus primaires de production de defaut.
Abstract. - The structures of colour centres in alkaline earth fluorides are now sufficiently well known to make studies of mechanisms of radiolysis possible. This short review describes results of recent measurements on the temperature dependence of the efficiency of F centre production in CaF2 and SrF2 and discuss primary processes of defect production.
I . Introduction. - A review of colour centres in alkaline earth fluorides has been prepared recent- ly [I]. Investigation of the structure of colour centres in these materials has now advanced to a stage where it is possible to begin meaningful studies of mecha- nisms of radiolysis. The F centre is well categorised in additively coloured crystals [I]. It is stable a t room temperature in the absence of light. The structure of the V, centre and the dynamics of its decay have been studied in detail [ I ] ; this centre in alkaline earth fluorides is stable to about 110 K. The recombi- nation luminescence of the V, centre has also been investigated [I]. The structure of the H centre (a hole trapped at a fluorine interstitial site) is known [I].
The hole is shared between the interstitial ion and a nearest lattice ion giving a ~nolecular axis alined along
<
1 1 1>.
This is in contrast with the VK centre which is alined along<
100>.
The H centre is stable in alkaline earth fluorides to about 170 K. It should be emphasized that in alkaline earth fluorides the H centre is more stable than the VK centre, the reverse of the situation in alkali halides.Irradiation of alkaline earth fluorides with 50 kV X-rays does not produce H centres in readily observable concentration. H centres are produced, however, for the same energy input of I MeV electrons. Because of this difference between efTects of purely ionising and of particle irradiation we shall divide our des- cription of effects of radiolysis into two sections, one dealing with X-rays and the other with electrons.
We shall not consider impurities directly here although the alkaline earth fluorides are known to
be quite impure. The role that impurities play in the radiolysis of these crystals remains to be established.
2. Experimental results. - 2 . I IRRADIATION WITH
X-RAYS. - 2.1
.
1 Stro~ltiu111 J L ( o I . ~ c / ~ . - The most detailed investigations of radiolysis carried out so far on alkaline earth fluorides have been made on SrF, [2]. Figure I shows the temperature dependence of the production of F centres in crystals of SrF, irradiated for 60 niin. in an optical cryostat using an OEG 60 X-ray tube operating at 50 kV and 40 mA.A pronounced fall in production rate occurs at 34 K and a further fall occurs at 67 K. When V, motion sets in at about 110 K the production rate falls to a very low value and does not recover a t temperatures up to 1500C. The ranges 4-34, 34-67
FIG. I. - Open circles : thermally induced annealing of the F band made by X-irradiating SrF2 at 30 K. The crystal was held at each point (except the lowest) for 5 min. and recooled
to 84 K for measurement. The heating-cooling cycles lasted I min. Closed circles: peak optical density of the F band produced
by X-irradiation for 60 ~nin. at dilferent temperatures. Conti- nuous lines are drawn thmugh the experimental points (after
Hayes and Lanlbourn [2]).
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973983
C9-500 W . HAYES
and 67-1 10 K will in future be referred to as ranges 1, I1 and Ill. It has been suggested [2] that these ranges are associated with the production of F - 1 pairs of increasing separation. V, centres are produced in concentration approximately equal to that of the F - 1 pairs ; H centres are not produced in observable concentration.
The F band produced by X-irradiation in range I shows partly resolved structure (Fig. 2a) and is quickly destroyed by bleaching with F band light at any temperature down to 4 K. This ready bleaching at 4 K may be a consequence of association of F centres with I centres. The destruction of the F band is accompanied by the same fractional destruction of the V, band. If the crystal is bleached in the low energy wing of the F band with linearly polarized light with the electric vector 8 parallel to [Oll] a readily observable dichroism results as well as an overall intensity reduction (Fig. 2a). Similar results may be obtained for the F band produced by irra- diation in range I1 (Fig. 2b). The sign of the dichroism in both cases is consistent with preferential destruc- tion of F - I pairs by excitation of electric dipoles oriented close to
<
1 I I>.
The shape of the F bandFIG. 2a. - Full line : F band produced by X-irradiating SrFz for 1 h a t 32 K. The crystal was subsequently bleached in the 500-600 nm region with linearly polarized light with E // [Oil]
and then the absorption was measured with light with E // [Oll]
(long dashes) and E // [ o i l ] (short dashes). All investigations were made at 32 K.
produced by irradiation in stage 111 is not appreciably different from that of stage 11 although it is slightly narrower.
The intensity of the F band produced by irradia- tion in range 1 does not change greatly with increasing temperature until the onset of V, motion at
-
110 K(Fig. I). The moving VK centres interact with F centres leading to the destruction of both and are partly retrapped at impurity sites forming V,, centres.
Heating to
-
175 K leads to liberation of holes from V,, centres and further destruction of F centres (Fig. 1). There is no change of sliape of the F band over the entire annealing range of figure 1. The thermal annealing curve for F cenlres produced by irradiation in range I1 is the same as for range I and again there is no change in sliape of the band with increasing temperature. Crystals warmed to room temperature after X-irradiation at low temperatures behave like virgin crystals presumably because of thermally- induced vacancy-interstitial recombination following conversion of F - 1 pairs to F + - I pairs by moving holes.It is found that if the F band is destroyed by optical bleaching at the temperature of creation a reirradiation for only a few minutes at the same temperature recreates the F band with intensity close to the original value. This holds for ranges I , 11 and I11 and shows that the F+ - 1 conlplexes resulting from the bleaching are stable. However, if we take a crystal through the following series of operations, 1) X-irradiate in range 1, 2) optically bleach the F band, 3) raise the temperature for a few minutes to range 11, 4) recool to range I and 5) reirradiate for a few minutes, we find an F band with the shape and intensity that would have been produced if the original irradiation had been carried out in range 11. A similar X-ray bleaching effect is produced if the above series of operations is carried out for ranges I1 and 111. This behaviour is consistent with the suggestion [2] that the ranges I, II and 111 are associated with F - I pairs of increasing separation and that the F band produced by irradiation in range I is a superposition of bands with relative intensities OD,, OD,, and OD,,, (Fig. 1).
Removing the contributions O D , , and O D I l , from the F band created in range I leaves the contribution OD, (Fig. 2c) which was assigned [2] to the nearest
FIG. 2c. - Residual F band shape obtained by subtracting FIG. 2b. - As for figure 2a except that X-irradiation was ODII
+
ODIII from OD1+
ODII+
ODIII (Fig. 1) (aftercarried out for 4 h at 42 K. Hayes and Lambourn [2]).
RADlOLYSlS OF A L K A L I N E EARTH FLUORIDES CY-501
F - I pair. This assignment is consistent with epr and endor data [2]. It sliould be emphasized, however, that impurities have not yet been ruled out as a source of perturbation of the F centre.
2 . 1 . 2 Catciunz Juorir/~>. - The colouration of CaF, at low temperatures by X-rays is qualitatively similar to that of SrFz [3]. The fall of F centre pro- duction efficiency with increasing temperature is shown in figure 3 ; there is a corresponding d r o p (not shown) in the production efficiency of V, centres.
Figure 3 also shows the thermally induced decay of the F band in CaF, after X-irradiation a t 44 K.
The loss between 84 and 150 K coincides with the decay of the VK centres. The decay beginning a t about 190 K is associated with the emptying of a n unidentified hole trap.
FIG. 3. - Closed circles : peak optical density of the F band in CaF2 produced by X-irradiation for 2 h at different tenipe- ratures. Open circles : thermally induced annealing of the F band in CaF2 produced by X-irradiation at 44 K. The crystal was held at each temperature above 84 K for 3 min. and recooled to 84 K for measurement. The heating-coolingcyclelasted I min.
(after Hayes and Lambourn [3]).
The results for CaF, (Fig. 3) are in contrast with those of SrF2 (Fig. I) which show sharp steps in tlie fall of production efficiency. I t appears that colou- ration in CaF, is not dominated by one close F - 1 pair.
The radiation-produced F band in CaF, is not as structured as in SrF, and there is little change in shape of the F band produced by irradiation a t different temperatures [3].
2 . 2 IRRADIATION WITH ELECTRONS. - Irradiation of CaF, and SrF, with I MeV electrons at 20 K results in the production of tlie epr spectra of F centres, H centres and VK centres 141. The production of V, centres saturates with a dose of about 2 x 10" elec- trons/cm2. However, H centre intensity increases linearly u p t o a dose of 1016 electrons/cm2, giving about one H centre for each incident electron. Anneal- ing to about 130 K, where the V, centres are mobile,
increases the H centre concentration by a factor of about 3 indicating that at least three times as many I centres as H centres are produced by the irradiation.
A quick reirradiation at 77 or 20 K restores the original H to V, ratio showing a radiation-controlled equilibrium between I centres and H centres. For the same input of X-ray energy at 20 K n o H centres are produced on warming to 130 K showing that X-rays d o not produce Isolated I centres.
The optical absorption spectra of CaF, and SrFz after heavy electron irradiation at 77 K shon a n intense absorption of V, centres (presumably super- imposed on a weak H band) (see [I]) and F centres [3].
Warming to 150 K for about 10 min. results in a drop in the intensity of the absorpt~on in the V, and F regions and a growth of a band due t o M + centres : this band occurs in CaF, at 552 nm and in SrF, at 658 nm. The conversion of F centres to M + centres is due to moving anion vacancies and it shows that a small fraction of the F centres produced by electron irradiation are well separated from interstitials. The concentration of H centres produced by the electron irradiation is approximately equal to the concentra- tion of M + centres that can be formed.
Whether o r not tlie well-separated vacancies and interstitials produced by electron irradiation are due to a knock-on process remains to be established.
Experiments using electrons with energies less than 0.5 MeV will be required.
3. Conclusions. - The F - 1 pairs produced by X-irradiation of alkaline earth fluoride crystals a t low temperatures are closely spaced and these mate- rials appear to be only on the verge of being colou- rable by X-rays. The fact that tlie crystals d o not colour under X-irradiation at room t e m p e r ~ ~ t u r c suggests that tlie potenlials afTecting interstitials in the vacancy-interstitial pairs are of a kind that result predominantly in thermally induced recombination rather than dissociation. Under 1 MeV electron irradiation at low temperatures a slnall fraction of the F - 1 pairs are well separated and these give rise to residual colouration on warming to room temperature. The nature of the primary event in the radiolysis of alkaline earth fluorides is not yet known and pulsed experiments [5] on these materials are desirable.
I n conclusion we emphasize that further detailed experimental work will be required t o determine the influence of impurities on the radiolysis of alkaline earth fluorides.
References
[ I ] HAYES, W. and STONEHAM, A. M., Ch. 4 of Ct.,~~.stal.s 11.it11 [3] HAYES, W. and LAMIIOURN, R. F., PIIJ's. Sful. SO/., (b) 51 thefirorire .str.rri~tro-c (ed. W . Hayes) (Oxford University (1973) 693.
Press) to be published in 1974. [4] STOTT, J. P., ~rnpublished work.
[21 HAYES, W. and LAMIIOURN, R . F., J . P/I!,S. C 6 (1973) 1 1 . [5] U ~ ~ T A , M., KONDO, Y., H I R A I , M. and Y ~ S H I ~ A I < I , T.,
J . Pli),.s. Soc. Jtrputr 26 ( 1969) 1000.
C9-502 W. HAYES
DISCUSSION
QUESTIONS BY H. W. DEN HARTOG Question. - Has the nearest-neighbour barium hyperfine interaction been measured for radiation- Question. - Could the high colourability be due induced centres in BaFz ?
to impurities such as oxygen ? Crystals of SrF2 Answer.. - No. It is important, however, that these produced at Jena with a low ionic conductivity in measurements should be made and we hope that the extrinsic region have a relatively low coloura- they will be carried out in the coming
bility.
Answer. - The results we described for SrF, QUESTION BY E. SONDER were obtained in crystals grown by Harsliaw, BDH
Question. - Electron irradiation usually deposits and our own laboratory. These crystals were clear
energy at a much greater rate than X-irradiation.
in the ultraviolet and the oxygen content is not
Did you check the possibility that the difference expected to be high. We feel that cationic impurities
between effects of X-ray and electron irradiations are more likely to be a problem. However, further
measurements such as the saturation density of F might be due to dose-rate effects ?
centres will be required to resolve this issue. The Answer. - The differences referred to were observed Jena results are of interest but we should like to for electron dose rates comparable with X-ray dose measure the colourability of the Jena crystals in our rates. Varying the electron dose rate by an order of own apparatus for direct comparison with our own magnitude did not produce noticeable dose-rate
crystals. effects.