G. Hauret

Two opposite effects of sodium on elastic constants of silicate binary glasses

(1 - x)SiO2-xNa2O glasses have been studied by Brillouin scattering when sodium oxide molar concentration x, varies in the range 0.05 < x < 0.44. Non-linear dependence of elastic constants on sodium oxide concentration is shown unambiguously for the first time and the compressive elastic constant C11 experienced a non-monotonic behaviour with a minimum value around x = 0.15. These results are compared with those obtained previously in the case of other alkali silicate glasses (Li, K). The variations of elastic constants of these glasses with the alkali metal nature and concentration are discussed on the basis of relations with the molar density of silicon atoms and the force constants between modifiers and non-bridging oxygens.

Elastic properties of sodium magnesium silica and sodium calcium silica glasses by brillouin scattering

Abstract In this article we present elastic constants measurements obtained by Brillouin scattering in sodium magnesium silica glasses. The results are compared with those known in sodium calcium silica glasses. Values and variations of elastic constants with the cationic Mg 2+ or Ca 2+ rate are discussed in term of middle range order.

Brillouin scattering in (NH4)2ZnCl4 above room temperature

Abstract A Brillouin investigation has been performed in (NH 4 ) 2 ZnCl 4 above room temperature. The investigated temperature range encloses the transition near 46°C andthe incomensurate phase between 91 and 133°C. Small anomalies of elastic constants appear at the transitions. They are qualitatively discussed.]

Similarities in the behaviour of magnesium and calcium in silicate glasses

Abstract The dependence of the infrared reflection spectra of mixed sodium-alkaline earth-silicate glasses on magnesium or calcium content shows similarities. This is surprising in view of previous observations indicating a possible network-forming role for magnesium. In these glasses, a well localized band for calcium or magnesium is observed at frequencies similar to those found in CaO or MgO. However calculation of the effective charge carried by each type of cation gives a degree of ionicity, Z / Z 0 , of 0.9 for calcium but only 0.7 for magnesium. This difference explains why magnesium might have some network-forming character in alumino-silicate glasses.

Brillouin scattering of high temperature phase transitions in LiKSO4

A Brillouin study of LiKSO4 has been performed in the temperature range 20–700°C. Discontinuous jumps in the temperature dependence of the Brillouin shifts appear at the transitions at 435°C and 670°C. In the intermediate phase (435°C < T < 670°C), two Brillouin modes are observed in the spectra of longitudinal acoustic waves propagating in the basal plane ([1 1 0] and [1 0 0] directions), suggesting the existence of crystallographic domains in this phase. The Brillouin spectra for [1 1 0] and [1 0 0] directions do not coincide in the intermediate phase, showing that this phase does not have the hexagonal symmetry, in disagreement with previous papers.

Brillouin light scattering in LiKSO4 between 20 and 80°C

Abstract Elastic constants of LiKSO4 crystals have been measured at room temperature and over the temperature range 20–80°C by Brillouin scattering. No anomaly has been found near 60°C, in disagreement with the results of a previous Brillouin study which suggested a new phase transition.

Dynamic behaviour of (1−x)SiO2-0.5xM2O glasses (M=Na, Li) investigated by infrared and Brillouin spectroscopies

Abstract Infrared reflectivity and Brillouin scattering investigations have been performed on alkali (Na, Li) silicate glasses of good optical quality, obtained from appropriate melts with cationic concentrations in the range 30–60%. The frequencies of vibrational modes have been extracted from fits of reflectivity spectra supplied by rapid-scan Fourier transform interferometry, while elastic constants were deduced from Brillouin scattering spectra obtained by high-resolution Fabry-Perot interferometry. Comparison of the results shows a correlation between the low-frequency vibrational cation mode and the elastic constant, C11, when the concentration or nature of the alkali cation change. An evolution of the covalent character of the bonds is displayed from measurement of the effective charge of oxygen.

Brillouin scattering in (NH4)2ZnCl4 near the lock-in transition

Abstract New Brillouin experiments have been performed in (NH 4 ) 2 ZnCl 4 above room temperature. The velocities of elastic longitudinal waves propagating along [0 1 1], [1 0 1] and [1 1 0] show small anomalies near the lock-in transition at 91°C. These results are discussed together with data which has been already obtained about elastic waves propagating along the crystallographic axes.

Phase transitions in the superionic protonic conductor CsHSeO4 investigated by Brillouin spectroscopy

Abstract A Brillouin investigation in CsHSeO 4 has been performed over the temperature range 20–165 °C which includes two phase transitions, in particular the transition to the superionic phase near T s = 129 °C. We observed strong discontinuities for elastic constants C 11 , C 22 and C 33 at T s and a broadening of the Brillouin lines above T s . The results are discussed on the basis of a linear coupling between strains and mobile protons.

Brillouin scattering by transverse acoustic waves in NH4 LiSO4 near the ferroelectric transition

The elastic constant C55 and the combinations (C44 + C55)/2 and (C55 + C66)/2 have been investigated as function of temperature from Brillouin scattering spectra by transverse acoustic waves. The main feature is an increase for every constant below the transition. A broadening of the Brillouin peak corresponding to (C44+ C55)/2 is also observed. The results are discussed on the basis of a biquadratic coupling between the shear strains and the order parameter.