Y. Luspin

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.]

Critical behavior of polar modes in lead phosphate near the ferroelastic phase transition

Abstract The temperature dependence of IR reflection spectra (15–4000 cm−1) of Au and Bu modes in the α phase and of Eu modes in the β phase of lead phosphate has been studied with a Fourier transform scattering interferometer. Frequencies, dampings and oscillator strengths of all modes show critical behavior, more or less marked, in the vicinity of the transition temperature. Correlations between the modes in both phases, including Raman results, are made. The temperature at which most of the modes lose their polar character is found more than ten degrees above that found by means of other techniques. Raman and X-ray data also indicate that characteristics of the monoclinic structure still persist well above Tc = 180°.

Soft mode spectroscopy in ABO3 ferroelectricss

Abstract A new interpretation of the phase transition mechanism, on the basis of recents results obtained by infrared reflectivity in ABO3 ferroelectrics, is discussed in terms of a crossover from displacive to order-disorder behavior. Deviation from the Curie-Weiss law in incipient ferroelectrics is also emphasized.

Brillouin spectroscopy in K2Ba(NO2)4

Abstract The temperature dependence of elastic stiffnesses of K 2 Ba(NO 2 ) 4 is investigated in the range - 140 to + 160°C, in which two phase transitions have been reported. Marked anomalies appear at the lower transition, in contrast with continuous variations through the upper transition. Results are compared to those obtained near the upper transition by an ultrasonic method and they are qualitatively discussed.

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.

Scanning infrared interferometry up to high temperature: A tool to obtain information on ferroelectric mechanisms and anharmonic couplings

Abstract The advantages of Fourier-transform infrared reflectivity spectroscopy with a scanning interferometer equipped to work up to 1400 K in the studies of ferroelectric materials are discussed. Results obtained in BaTiO3 are compared with Raman data. Anharmonic effects such as anomalous dependence of damping on temperature are briefly analysed.

Hydrostatic pressure effect on commensurate-incommensurate transition in K2SeO4-elastic anomalies studied by Brillouin scattering

Abstract A Brillouin scattering study of the commensurate—incommensurate transition in K2SeO4 submitted to a hydrostatic pressure was performed. It was found that an increasing pressure lowers the transition temperature (dTi/dPi = −59 K/GPa). The strong anomalies of velocity and damping of the longitudinal acoustic wave propagating along [0 0 1] are weakly affected when pressure acts. The effect of pressure on these anomalies has been calculated on the basis of the lowest order couplings eQ2 and e2Q2 between strain e and order parameter Q.

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.

Anomalies of brillouin spectra in NH4LiSO4 near the ferroelectric transition

The longitudinal elastic wave propagating along [010], corresponding to the elastic constant C22, shows a strong anomaly of damping near the upper transition (Tc ≃ 188°C). An additional anomaly of peak intensity of the Brillouin line has been found below the transition and seems not to be related to the elastic anomaly. This one has been explained on the basis of couplings between the strain and the order parameter. The susceptibility associated with the order parameter is found to have a relaxational behaviour characterized by a relaxation time τ = A(T - T1)-1 with A = 1.1 × 10-11 s · K and with T1 ≃ Tc.

Stabilization of the soft mode in the paraelectric phase of BaTiO3

Abstract Infrared reflectivity measurements are performed on single crystals over a wide temperature range in the cubic phase. A stabilization of the over-damped lowest-frequency mode is observed and the temperature dependence of the frequency is consistent with a Curie law behaviour. This result, in agreement with Cochrans soft mode theory, confirms the displacive aspect of the phase transition. The temperature dependence of the damping coefficient is also presented.