Temperature investigations of the ferroelectric crystals Sn2P2S6 and Sn2P2Se6 by means of 119Sn Mossbauer spectroscopy

Mossbauer spectra of Sn2P2S6 and Sn2P2Se6 proper uniaxial ferroelectrics were investigated over a wide temperature region including phase transition points with the aim of studying the dynamics of ferroactive tin ions used as a Mossbauer isotope (Sn-119). The characteristics of the gamma-resonance lines (isomer shift, effect probability, etc) and the parameters calculated from the measured spectra in the Debye approximation (mean square displacement, Debye-Waller factor, Debye temperature, etc) were analysed in comparison with the available structural and thermodynamical data. The data obtained support the assumption of a two-well local potential of the tin atoms in addition to the displacement of their time-averaged position below the phase transition points. It was concluded that displacement and ordering occur in the same Sn sublattice, and that the variation in long-range forces leads to lattice instability.