Eur. Phys. J. B 5, 169-178 (1998)
https://doi.org/10.1007/s100510050431

Soft modes and phonon interactions in Sn₂P₂S₆ studied by neutron scattering

¹ Research Institute for Materials, University of Nijmegen, Toernooiveld, 6525 ED Nijmegen, The Netherlands
² Institut Laue-Langevin, BP 156X, 38042 Grenoble Cedex, France
³ CNRS Laboratoire de Cristallographie, BP 166, 38042 Grenoble Cedex, France
⁴ Université Toulon-Var, BP 132, 83957 La Garde Cedex, and CEMES (UPR CNRS 8011.) , BP 4347, 31055 Toulouse Cedex, France
⁵ Laboratoire Léon Brillouin, CEA/Saclay, 91191 Gif-Sur-Yvette, France
⁶ Institute for Physics and Chemistry of Solid State, Uzhgorod State University, 294000 Uzhgorod, Ukraine

Corresponding author: ^a eijt@labs.polycnrs-gre.fr

Received: 11 May 1998
Accepted: 15 June 1998
Published online: 15 September 1998

Abstract

The low frequency lattice dynamics and its relationship to the second order paraelectric-to-ferroelectric transition in Sn₂P₂S₆ is studied. The dispersion branches of the acoustic and lowest lying optical phonons in the a^*-c^* plane have been obtained in the ferroelectric phase, for x-polarized phonons. Close to the phase transition a considerable softening is found for the lowest optical mode (P_x), comparable to the behaviour observed in previous Raman investigations. As found previously in Sn₂P₂Se₆, a strong coupling between the TO(P_x) and TA(u_xz) phonons is observed, although, apparently, not strong enough to lead to an incommensurate phase. The soft TO(P_x) mode at the zone center is observed. The temperature dependence of its frequency and damping shows that the transition is not entirely displacive. At low temperatures an unusual apparent negative LO-TO splitting is observed which is shown to arise from the coupling of the x-polarized soft mode to the nearby z-polarized optical phonon. For comparison, the soft TO(P_x) dispersion in the a^*-b^* plane is measured in both the paraelectric and ferroelectric phases. Consistent frequency changes and LO-TO splitting are observed, revealing a significant interaction between the TA(u_yx) and LA(u_xx) acoustics branches and the TO and LO soft optic branches, respectively. In contrast, the nearby y-polarized optic branch shows almost no temperature dependence. Finally, the influence of piezoelectric effects on the limiting acoustic slopes in the ferroelectric phase is discussed.