SOFT MODES AND PHONON INTERACTIONS IN SN2P2SE6 STUDIED BY MEANS OF NEUTRON-SCATTERING

This paper presents an inelastic neutron scattering study of the prope r ferroelectric Sn2P2Se6 and elastic neutron scattering results on the satellite diffraction pattern which characterizes the modulated phase . The temperature dependences of the satellite intensities and modulat ion wavevector are in fair agreement with results from previous x-ray experiments. Close to the incommensurate-to-ferroelectric transition t emperature T-c, an unexpected intensity overshoot is observed, similar to that seen in birefringence and dilatation experiments. The relatio nship between the lattice dynamics and the observed phase transition s equence is examined. The dispersion of the ferroelectric soft optic ph onon (P-x-polarization) and of the acoustic phonons is followed along the a- and c*-directions. In the ferroelectric phase, the TO mode sho ws a considerable softening as the incommensurate phase is approached from below. In the paraelectric (T > T-i) and incommensurate (T-c < T < T-i) phases, the response from the TO (P-x-polarization) and TA (par tial derivative u(x)/partial derivative z strain) branches has been in vestigated via a series of constant-q scans in the c-direction (appro ximately the modulation wavevector direction). The combined inelastic line-shapes, as observed in a number of non-equivalent Brillouin zones , could all be analysed in terms of a coupled-mode damped harmonic osc illator model. In addition, a diverging, resolution-limited, central p eak is observed close to T-i. It is suggested that the TO-TA coupling lies at the origin of the incommensurate instability. A phenomenologic al free energy is developed, in the continuum approximation, in which the TO-TA interaction is included via a pseudo-Lifshitz term of the ty pe (partial derivative u(x)/partial derivative z)(partial derivative P -x/partial derivative z).