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Submitted on 1 Jan 1980
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Raman study of V-centres in X-irradiated KI and RbI
S. Lefrant
To cite this version:
Raman study of V-centres in X-irradiated KI and Rbl
Abstract. — Raman experiments were carried out on KI and Rbl, X-irradiated over a range of temperature for which V-centres are created. After X-irradiation at 190 K, a Raman band was observed at 112 cm"', assignable to the symmetric stretching vibration of an I3 molecular ion. Another band is observed at 173 cm- i (160 cm"* for
Rbl) which could be due to « I2 » molecules in a site which cannot be precised. After X-irradiation at RT of KI, the
Raman spectrum is composed of two lines at 180 and 189 cm "', probably due to large iodine aggregates.
1. Introduction. — The structure of V-centres creat-ed in alkali halides by exposure to X-rays at tempe-ratures higher than 100 K is still unknown. These centres are not paramagnetic and only absorption data are available in the literature. The V-centres we are concerned with give rise to an absorption band in the U.V. spectral range and previous studies of optical dichroism [1] have shown that their orien-tation is presumably [100]- Several models have been suggested for these centres :
— A molecular ion XJ (X is an halogen atom) as proposed by Hersh [2] for V-centres in KI by compa-rison with absorption of IJ ions in aqueous solutions.
— An « X2 » molecule, the two atoms occupying either two cube-centre sites as proposed by White and Greene [3] in NaCl or accommodated in a cation-anion vacancy pair as proposed by Hobbs et al. [4].
It has also been shown that in KBr, two H-centres transform into one di-H-centre [5] by thermal anneal-ing. A similar observation has been made in SrCl2 by Rzepka et al. [6] where two Vk-type centres form an « E » centre, with similar features and attributed to molecular complexes.
In order to elucidate the V-centre problems, we used the Raman technique, in conjunction with the optical absorption studies, on KI and Rbl crystals. The V absorption bands are close to the visible spectral range, and Raman spectra of reasonable intensity can be obtained using the blue lines of a cw Ar+ laser. We have performed X-irradiations at different
(*) Equipe de Recherche associee au C.N.R.S., n° 13.
temperatures and the most typical results are described below.
2. Results. — 2.1 KI. — After X-irradiation at 190 K (Fig. lb), in addition to a very strong absorption band, another band is observed at 335 nm with a shoulder at 275 nm (a further shoulder is sometimes present at 375 nm but this is sample dependent). The Raman spectrum obtained under these
condi-Fig. 1. — Absorption spectra at T = 10 K of KI X-irradiated : a) at T = 10 K (right-hand ordinate, broken curve); 6) at
T = 190 K (left-hand ordinate, full curve); c) Raman spectrum at
r = 78 K of KI X-irradiated at 190 K. /.„„, = 488 nm. The smaU
features labelled F arise from F-centres.
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 7, Tome 4 1 , Juillet 1980, page C6-476
S. Lefrant and E. Rzepka
Laboratoire de Physique Cristalline (*), Université de Paris-Sud, Bâtiment 490, 91405 Orsay-Cedex, France
Résumé. — Nous avons effectué des expériences de diffusion Raman sur Kl et Rbl, irradiés aux rayons X dans une
gamme de température telle que des centres V soient créés. Ces centres n'étant pas paramagnétiques, les seuls résul-tats disponibles sont ceux de l'absorption optique. Après irradiation à 190 K, nous avons observé en diffusion Raman une bande à 112 c m "! attribuable à la vibration de valence d'un ion moléculaire I 3 . On observe une autre
bande à 173 c m "l (160 c m "1 pour Rbl) qui pourrait être due à la présence de molécules « I2 » dont le site n'a pu
être précisé. Enfin, après irradiation à 20 °C de Kl, le spectre Raman est composé de deux raies à 180 et 189 c m "1,
dues vraisemblablement à de gros agrégats d'iode.
RAMAN STUDY O F V-CENTRES IN X-IRRADIATED K1 AND Rbl C6-477 tions (Fig. lc) is essentially composed of two broad
bands at 1 12 and 173 cm-
'.
- X-irradiation at 273 K leads to a weaker absorption band at 335 nm and an unresolved band appears a t 310 nm (Fig. 2a). The Raman bands observed previously are still present but their intensity ratio is reversed (Fig. 26).
Fig. 2. - cr) Absorption spectrum at T = 10 K of K1 X-irradiated at 273 K. b) Raman spectrum of the same sample at T = 78 K.
A,,,,, = 488 nm.
- After X-irradiation at 293 K, the absorption spectrum is completely different (Fig. 30). Absorption in the V-centre region changes from the visible to the U.V. spectral range with unresolved peaks. In Raman scattering, two sharp and strong lines are observed at 180 and 189 cm-
'
respectively (Fig. 3b). These lines can also be observed as very weak struc- tures in a sample X-irradiated at 190 K and stored for a long time in the dark at room tempe- rature (Fig. 4). In the same way, experiments on crystals coloured by addition of an excess of iodine at 550 OC reveal the formation of centres responsible of the same Raman lines.K I X - ~ r r a d ~ a t e d a t 140K A = d s s o A ~ ~ 7 8 ~ 112 - A f t e r lrradlatlon --... Stored 15 days at RT m
-
<
In z W F 2 I Q m I I # I 0 50 100 150 200 cm-' Flg. 4. - Raman spectrum of KI T = 78 K ; i .,,,,, = 488 nm.a ) After X-irradiation at 140 K , b) After storage 15 days in the dark at RT.
2 . 2 RbI. - After X-irradiation at 100 K, we mainly observe a Raman spectrum similar to that previous12 obtained for KI, i.e. two bands at 113 and 160 cm-' (Fig. 5). Nevertheless, as the irra- diation temperature is increased, the 160 cm-' Raman band decreases in intensity. If the irradiation is performed at room temperature, only the 113 cm-
'
band remains in the Raman spectrum (Fig. 6). In the V-centre absorption region, only the V,,, band is observable a t 350 nm.Fig. 3 . - a ) Absorption spectrum at T = 10 K of KI X-irradiated I , I I
50 100 1 M 2W c m - 1
at 293 K. F, R,, R, and M denote the absorption bands of the
corresponding centres. b) Raman spectrum of the same sample Fig. 5. - Raman spectrum of RbI, X-irradiated at 100 K.
C6-478 S. LEFRANT AND E. RZEPKA
R ~ I X.~rrad~ated a t 2 9 3 ~ attribution of the V3,, band in KI and V,,, band in
1
RbI to I i centres is confirmed. Unfortunately, it has not been possible to determine the orientation of such a centre by the Raman technique but a recent model [lo] in which the I; ion occupies two anion and one cation sites seems reasonable (that was already proposed for the V3-centre in KC1 by Christy and Phelps [l 11).The Raman lines at 180 and 189 em-' observed after X-irradiation at room temperature of KI are similar to those seen in molecular 1, crystals [12] and show the same resonant behaviour. This result is compatible with the formation of large aggregates of iodine as already observed in irradiated or halogen Fig. 6. - Raman spectrum of Rbl, X-irradiated at 293 K. doped samples [ l q . NO aggregates of this type were T = 78 K, i,,,,, = 488nm (the absorption spectrum is Shown detected in RbI since molecular ions I; are stable in the inset top right).
until 70 OC. An RbI crystal additively coloured with an excess
of iodine reveals the 113 cm-' Raman band when treated at 550 O C and both 1 13 and 160 cm -
'
bandswhen treated at a lower temperature (400 OC).
3.. Conclusion. - The first result which comes out of
this study is the correlation between the absorption band V,,, in KI (or V3,, in RbI) and the 112 cm-' Raman band (1 13 cm-' for RbI). It is well known that the molecular ion I; has a syr?7t?letric stretching vibration at 1 1 1 cm-
'
when prepared in solu- tions [7, 81. This molecular ion has also been observed recently in some doped polymers [9]. Then, theInterpretation of the 173 cm-
'
Raman band is not straightforward. Nevertheless, the possibility cannotbe excluded that an cr I, n molecule is formed. This molecule is expected to show absorption in the range 450-500 nm as confirmed by our optical data after bleaching experiments were performed in this region. Even if these centres are present at a low concen- tration, the use of the 488 nm excitation line could enhance the 173 cm-' Raman band by resonance effects. The question remains about the sites occupied by the halogen atoms. Location in a divacancy appears as reasonable if a conversion occurs between this (( I, )> molecule and a molecular ion I; in [loo] orientation.
DISCUSSION
Question.
-
N . ITOH.You mentioned that the bending mode was not observed. Can you estimate the possible upper limit of the bending angle from the absence of the bending mode ?
Reply.
-
S. LEFRANT.It is not possible to estimate an upper limit for a bending angle of the I; molecular ion as the bending
mode, when observed, is always weak. The fact that only the stretching vibration is observed is rather in favor of a linear molecular ion.
Comment.
-
F . AGULLO-LOPEZ.I am happy that you have found new experimental evidence for the occurrence of an X; centre. It appears that this type of centre is, indeed, required to explain our recent thermoluminescence data on NaCl : Mn.
References
[I] OKUDA, A., HARAMI, T., OKADA, Y. and TAKEI, O., J. Phys. Soc. Japan 43 (1977) 993.
[2] HERSH, H. N., Phys. Rev. 105 (1957) 1410.
[3] WHITE, W. W. and GREEN, A. C., Cryst. Lattice Defects 1
(1969) 83.
[4] HOBBS, L. W., HUGHES, A. E. and POOLW, D., Proc. R. Soc. London A 332 (1973) 167.
[5] SAIDOH, M. and ITOH, N., J. Phys. Chem. Solids 34 (1973) 1165.
[6] RZEPKA, E., LEFRANT, S. and TAUREL, L., J. Phys. C : Solid St. Phys. 8 (1975) 2523.
[7] KIEFER, W. and BERSTEIN, H. J., Chem. Phys. Lett. 16 (1972) 5.
[8] KAYA, K., NIKAMI, N., UDAGAWA, Y . and TO, M., Chenz. Phys. Lett. 16 (1972) 151.
[9] LEFRANT, S., LITCHMANN, L. S., TEMKIN, H., FITCHEN, D. B., MILLER, D. C., WHITWELL, G. E. TI and BURLITCH, J. M.,
Solid State Commun. 29 (1 979) 191.
[lo] ELANGO, A. A. and NURAKHMETOV, T. N., Phys. Status Solidi (6) 78 (1976) 529.
1111 CHRISTY, R. W. and PHELPS, D. H., Phys. Rev. 124 (1961) 1053. [12J ANDERSON, A. and SUN, T. S., Chem. Phys. Lett. b (1970) 61 1. [13] HOBBS, L. W., J. Physique Colloq. 37 (1976) C7-3 (and references
