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AGGREGATION OP POSITIVE HOLE CENTRES IN
SrCl2, DOPED WITH ALKALI CATIONS
E. Rzepka, S. Lefrant, L. Taurel
To cite this version:
C7-230 JOURNAL DE PHYSIQUE Colloque C7, supplkment au no 12, Tome 37, Dkcembre 1976
AGGREGATION OF POSITIVE HOLE CENTRES
IN SrCI, DOPED WITH ALKALI CATIONS
E. RZEPKA, S. LEFRANT and L. TAUREL Laboratoire de Physique Cristalline (*), UniversitC de Paris-Sud,
BItiment 490, 91405 Orsay Cedex, France
R6sum6. - La formation d'agrkgats de centres A trous positifs (appelks centres E) est CtudiQ dans SrClz : M+ (M = Na, K, Rb) irradik aux rayons X entre 130 et 230 K. Les rksultats montrent
que la bande E est formee a partir de deux centres a trous positifs.
Les proprietks essentielles des centres E sont dkcrites et comparkes a celles des di-interstitiels dans les halogknures alcalins. Les analogies entre les propri6t6s de ces centres permettent d'avancer l'hypothkse d'une structure similaire, ces centres pouvant Ctre des complexes quasi-molkculaires. Abstract. - Formation of aggregates of hole centres (called E centres) in SrC12 : M+ (M = Na,
K, Rb) by X-irradiation between 130 K and 230 K is studied. The results show that the E centre is formed from two positive hole centres. The main properties of the E centres are described and compared to those of the di-interstitial centres in the alkali-halides.
The analogy between the properties of these centres supports the hypothesis of similar structure, these centres likely being quasi-molecular complexes.
1. Introduction. - It is now well known that SrCl, can be coloured by X-rays at temperature lower than RT only when the crystal is doped with impurities such as Na', K C , Rb' [I].
When the X-irradiation is performed at 10 K, the absorption spectrum consists of a broad band A
(A
= 399 nm) due to the '2: 4'c:
transition of the Vk centres [2] and of a double band B which has been attributed to F centres perturbed by the monovalent cation [3]. Paramagnetic resonance studies of Vk centres show spectra whose features are independent of the nature of the impurities ( I ) .When the crystal is X-irradiated at T
>
130 K, VkA centres are created. The A band is slightly shifted towards short-wavelengths and modifications appear in EPR spectra according to the nature of the mono- valent cations [4]. Moreover, a new centre (called E) responsible of an E band at 263 nm appears, and our purpose is to indicate its mains properties which are very close to those of the V, centres in alkali-halides [5]. 2. Formation and properties of E centres. - If we examine the condition of the E centres formation, it appears that these centres are always created when the V,, centres are thermally unstable. In particular, the best condition consists of an X-irradiation in the range 200-230 K ; the concentration of VkA is very small, whereas that of electron centres can be veryimportant. These results lead to the assumption that the E band is due to positive hole centres.
During X-irradiation at a given temperature in the range 130-230 K, the optical density of the E band and that of the A band are related by the quadratic relation a(,, = Kg2(,, with K = KO exp(- U/kT), (U = 0.12 eV). This suggests that an E centre is formed from two positive holes. This model explains that paramagnetism due to E centres has never been detected. Under X-irradiation a positive hole may be trapped by a VkA centre, so that the charge deficiency on the cation impurity decreases the electrostatic interaction between the two positive holes and the process of aggregation may exist.
The dichroism induced at LNT in the E bands shows that the centre has a four-fold symmetry axis. This centre reorients thermally at 160 K with an activation energy equal to 0.4 eV. The main properties of the E centre can be compared to those of the V(240) centres in KC1 [6], or V4 in KBr [7, 81 or V(,,,, in KI [9] : thermal stability (230 K), orientation under optical excitation at 80 K, thermal reorientation (160 K) and photodestruction at 10 K [5] are very similar to those of di-interstitial centres.
Moreover, the energy of the absorption band of the E (4.71 eV) and V(240) (5.32 eV) are in the same spec- tral region. The half-widths of these bands are res- pectively 0.685 and 0.75 eV at 13 K.
(*) Equipe de Recherche associCe au C. N. R. S., no 13, Likewise, very similar laws account for the variation (1) EPR spectra cannot be measured at T < 16 K because of the second IIloment of the E band with the tempe- a saturation occurs. rature. In the case of E centres, the results agree with an
AGGREGATION OF POSITIVE HOLE CENTRES IN SrClz DOPED WITH ALKALI CATIONS C7-231
effective frequency 0,/2 n equal to 390
+
10 cm-I. Moreover, our analysis of the V(,,,, band in KC1 leads to an effective frequency of 403 _+ 15 cm-I. In both cases, these frequencies are higher than the highest frequencies of the lattice phonons (243 cm-' in SrC1, and 210 cm-' in KC1).These results suggest that the E and
V(240)
centres are quasi-molecular complexes and that the effective frequency accounts for the normal modes of vibra-tion of the quasi-molecule. Then, we can expect that this frequency is less sensitive to the structure of the host lattice than to the nature of the halogen ion and that it decreases when the halogen mass increases. Indeed, our analysis of the V4 absorption band in KBr and V(330) in KI leads to results in good agree- ment with this suggestion, since the value of 0,/2 n are respectively 257 8 cm- ' and 175 f 15 cm- in KBr and KI.
References
BALTOG, I., LEFRANT, S., HOULIER, B., YUSTE, M., CHA-
PELLE, J. P. and TAUREL, L., Phys. Status Solidi (b) 48 (1 971) 345-52.
-
-
RZEPKA, E., MITROAICA, G., LEFRANT, S. and TAUREL, L.,Solidstate Commun. 13 (1973) 1499-501.
(31 RZEPKA, E., BALTOG, I., LEFRANT, S., YUSTE, M. and TAUREL, L., Phys. Status Solidi (b) 57 (1973) 383-92.
(41 The lack of resolution of s.h.f. structure in EPR spectra of <( Vk >> centres formed by X-irradiation at 78 K [I]
is due to a mixing of Vk and VBA centres.
[5] RZEPKA, E., LEFRANT, S. and TAUREL, L., J. Phys. C : Solid State Phys. 8 (1975) 2523-31.
[6] C ~ r s m , R. W. and PHELPS, D. H., Phys. Rev. 124 (1961) 1053-60.
[7] SAIDOH, M. and ITOH, N., J. Phys. & Chem. Solids 34 (1973) 1165-71.
[8] HOSHI, J., SA~DOH, M. and ITOH, N., Cryst. Lattice Defects
6 (1975) 15-20.
[9] KONITZER, J. D. and HERSH, H. N., J. Phys. & Chem. Solids
27 (1966)771-81.
DISCUSSION
A. NOUAILHAT.
-
What is the luminescence corn- does not give any luminescence due to recombina- portment of the E-center if you release electrons from tion with E centres (at LNT or LHeT).traps ?
2) No large excitation with electron was made. But Can you make with large excitation, bi-excitation
