LIGHT SCATTERING FROM ACOUSTIC AND LOW-FREQUENCY VIBRATIONAL MODES IN BORATE GLASSES

Abstract

Brillouin and low Raman scattering experiments have been performed on series of B203 xM20 glasses where = Li, Na, K, Rb, Cs, 11. The concentration and composition dependences of the velocity and attenuation of longitudinal and transversal acoustic waves have been measured, as well as those of the Raman peak. The composition and concentration dependence of the acoustic velocity seems to result from two competitive effects : the first one is related to the transition of boron atoms from three to four coordination with increasing x and the second one to the structural changes connected to the sterical hinderance of the M cations. The maximum of the boson peak has a behaviour similar to that of sound velocity. This similarity can be understood on the basis of the continuum theory of low-frequency Raman spectrum. Introduction. The binary borate glasses B203 x M20 are currently used as vitreous matrices for the elaboration of ionic conductors. In order to understand the conduction mechanisms in ternary systems (e.g. B203 xLi 20-y LiC£ or B203 x Li20yLi2S00 (7,2,3), it is necessary in a first step, to get structural informations on the binary matrix. For this purpose we have used the Brillouin and Raman scattering techniques. The identification of the structural units of the glass can be made qualitatively from the analysis of the high frequency part of the Raman spectra, while Brillouin experiments can give connection between the acoustical and structural properties. Brillouin results are also of first interest for the interpretation of the so-called Boson peak which dominates the low part of the Raman spectra. Following the Martin and Brenig model (4) the structural correlation range, which reflects the average size of the boron oxygen edifices, can be deduced. High Raman data. The amorphous B203 x M20 systems have already been widely studied by Raman spectroscopy 11,5,6). This spectrum is dominated by two lines at 806 cm and 780-765 cm respectively due to the breathing vibration of boroxol and of six-membered rings containing BO., units (di tri tetraborate groups). By a systematic study of all alcali borate glasses (and also T£20 containing glasses) we have found different laws for the progressive disappearance of the boroxol 806 cm line with increasing values of x : the larger the ionic radius of M, the earlier the 806 cm line disappears. This is compatible with an Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19829108 C9-542 JOURNAL DE PHYSIQUE increase of the dimensions of the borate edifices with increasing s izes of M+ and can be interpreted from stoechiometric reasons by the dominant formation of tribor a t e groups i n the case of B2O3-x Li20 , and tetraborate groups for B203 X Cs20 (0 0.2) 20 does not exceed the dimension of one six-membered ring (this is for instance the case of tr iborate groups). A s the mass of the atom increases, 20. also increases ; t h i s agrees with the formation of borate groups containing more than one six-membered rings. A detailed analysis of these resul ts w i l l be published i n a subsequent forthcoming paper. Fig.2 Low Raman s p e c t r a of Fig .3 S t r u c t u r a l c o r r e l a t i o n range B203-0 ,25 M20 g l a s s e s deduced from the Martin-Brenig t h e o r e t i c a l f i t s wi th Martinmodel.