Lattice dynamics and the ferroelectric phase transition in Sn2P2S6

This paper presents a quasiharmonic model for the lattice dynamics of the S n2P2S6 crystal. The model, constructed as a combination of the valence forc e field and the polarizable ion model with a majority of potential paramete rs fixed by auxiliary Hartree-Fock calculations, enables us to calculate vi brational spectra for both the paraelectric and the ferroelectric structure . The calculated eigenvector of the lowest frequency optic mode corresponds to the order parameter of the ferroelectric phase transition and can be id entified with the symmetry-breaking soft mode. Interpretation of the obtain ed results and their comparison with available experimental data is provide d.