Abstract
We present an inelastic neutron scattering study of the relationship between lattice dynamics and the phase transition sequence for the proper ferroelectric Sn2P2Se6, which demonstrates a type II incommensurate phase. The dynamics in the phase transition sequence paraelectric-incommensurate-ferroelectric shows the following evolution. The TO (P x ) phonon softens in the paraelectric phase. Close to the incommensurate phase near the modulation wave vector position [qvec]IC (T i ) a central peak appears accompagnying the condensation of the TO mode. In the incommensurate phase the amplitudon branch is observed, which increases in frequency upon lowering the temperature. In the ferroelectric phase the soft mode in the zone center shows a clear hardening upon lowering the temperature. In the paraelectric and incommensurate phase the TO (P x polarization) and TA (u xz strain) phonon responses appear as a combined interference line shape for propagation direction c*, i.e. approximately along the modulation wave vector. We suggest that the coupling between the TO and TA phonons is at the origin of the incommensurate phase formation.
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