LATTICE-DYNAMICS OF LITHIUM-INTERCALATED IRON PHOSPHORUS TRISULFIDE

We review the recent investigations on the lattice dynamics of lithium-intercalated FePS3. These studies represent an extension of those made for the pure material and have been performed by means of a force-constant model generated by a set of short-range two-body potentials. The intercalated phases have been investigated for the three stoichiometric compositions Li0.5FePS3, LiFePS3 and Li1.5FePS3 with the aim of analysing the evolution of the host lattice normal modes as a function of the concentration, and finding the dispersion of the new phonon branches induced by lithium. The above special values of lithium concentration have been chosen because the size of the unit cell remains the same as in the host material. The interaction between lithium and sulphur includes only nearest neighbours and is also described by a two-body potential. The Li-Li interaction is described by a Born-Mayer potential. The choice of this simplified model is dictated by the limited number of experimental data: these are the Raman and infrared data for the host material and only the infrared data for the intercalated phases. The force constants of lithium are fitted to these data and the phonon dispersion curves are calculated along the symmetry directions of the Brillouin zone. A spectroscopic method for monitoring lithium migration in the host material is proposed. © 1989.