Lattice dynamics and phase transitions in Sn_{2}P_{2}S(Se)_{6} ferroelectric crystals

Abstract

Polarizable ion model was used in describing the lattice dynamics in the Sn 2 P 2 S 6 and Sn 2 P 2 Se 6 ferroelectric crystals and their solid solutions. There has been obtained a good agreement of the calculated phonon spectra with the experimental data of Raman and Brillouin spectroscopies, neutron scattering and ultrasound experiments. Paraelectric phase instability has been modelled by negative charge transferring from the tin cations to the chalcogen ions, and by increasing the polarizabilities of the latter. The ferroelectric phase has been stabilized by tin atom displacement from centrosymmetric positions and a partial reversal recharging between tin and chalcogen ions. The effective charges decreasing by moving from Sn 2 P 2 S 6 to Sn 2 P 2 Se 6 compounds determine the shift of condensation wave vector of soft optic phonons in paraelectric phase from q = 0 in sulfur compound to q > 0 in selenium compound. Such a change of dynamical instability character is connected with the appearance of incommensurate phase in Sn 2 P 2 (SexS 1−x ) 6 solid solution under x > 0.28. The "two-mode" instability emerged as possible in the vicinity of Lifshitz point -there have been modelled the condensation of soft optic mode with q ≈ 0 and mixed optic and transversal acoustic phonons near q > 0. This fact requires further experimental and theoretical investigations.

Acoustic attenuation in ferroelectric Sn2P2S6 crystals

“…The thermal diffusivity and thermal expansion were studied, and the crystal anharmonicity was characterized [15][16][17]. Ultrasound spectroscopy and Brillouin scattering data for Sn 2 P 2 S 6 crystals and Sn 2 P 2 (Se S 1− ) 6 solid solutions have been presented in papers [18][19][20][21][22]. In the ferroelectric phase, the temperature dependence of ultrasound velocity was described in Landau-Khalatnikov (LK) approximation [23].…”