Lattice dynamics and phase transitions in Sn2P2S(Se)6 ferroelectric crystals

Polarizable ion model was used in describing the lattice dynamics in the Sn2P2S6 and Sn2P2Se6 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 Sn2P2S6 to Sn2P2Se6 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 Sn2P2(SexS1-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 approximate to 0 and mixed optic and transversal acoustic phonons near q > 0. This fact requires further experimental and theoretical investigations.