Acoustic attenuation in ferroelectric Sn2P2S6 crystals

Abstract

Abstract:The temperature and frequency dependencies of sound attenuation for the proper uniaxial ferroelectric Sn 2 P 2 S 6 , which has a strong nonlinear interaction of the polar soft optic and fully symmetrical optic modes that is related to the triple well potential, were studied by Brillouin spectroscopy. It was found that the sound velocity anomaly is described in the Landau-Khalatnikov approximation with one relaxation time. For explanation of the observed temperature and frequency dependencies of the sound attenuation in the ferroelectrric phase, the accounting of several relaxation times is needed and, for quantitative calculations, the mode Gruneisen coefficients are more appropriate as interacting parameters than are the electrostrictive coefficients. Relaxational sound attenuation by domain walls also appears in the ferroelectric phase of

Description of lattice anharmonicity observed in ferroelectrics with unusual three-well local potential

“…and with the assumption that the volume changes are proportional to the order parameter variation (or average displacement of oscillators). Such an approach yields negative values of the mode Grüneizen parameters of ω 01 and ω 12 excitations (see figure 4) which coincides with the previously estimated contributions into thermodynamic Grüneizen parameters of Sn 2 P 2 S 6 ferroelectric phase -negative for optic modes and positive for acoustic modes [19]. However, this qualitative coincidence of thermodynamic and spectral characteristics is insufficient for quantitative characterization of the crystal lattice anharmonicity.…”

“…The interaction between these excitations is demonstrated by their intensities redistribution [see figures 2(b), 3(a), 3(b)]. At approaching the transition temperature T 0 at an ambient pressure, many low-energy excitations appear, and the experimentally observed spectrum contains very damped asymmetric lines combined with a relaxational central peak [19]. Baric transformation of pseudospin fluctuations spectrum, at approaching the transition pressure about 0.24 GPa [see figure 3(b)], similarly demonstrates a clear softening of two highest frequency excitations and their interference with many new low-energy spectral peculiarities.…”

Ferroelectricity in (PbySn1−y)2P2S6 mixed crystals and random field BEG model

“…This overestimation can be corrected by taking account of several relaxation times and mode Gruneisen coefficients. These coefficients are more appropriate as the interaction parameters than the electrostrictive coefficients, as was earlier discussed in details for pure Sn 2 P 2 S 6 crystals [21]. On cooling in ferroelectric phase, additional sound attenuation by domain walls can be observed as well [21].…”

Flexocoupling-induced soft acoustic modes and the spatially modulated phases in ferroelectrics

“…Basic experimental methods, which contain information about the soft modes and spatial modulation of the order parameter in ferroics (such as antiferroelectrics, proper and incipient ferroelectrics) are dielectric measurements [11], inelastic neutron scattering [5,12,13,14,15,16,17], X-ray [18,19,20,21], Raman [22] and Brillouin [8, 18,21,23,24,25] scatterings and ultrasonic pulse-echo method [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] allowing hypersound spectroscopic measurements.…”