Lattice dynamics of pure and lithium-intercalated transition-metal phosphorous trichalcogenides

The authors investigate the vibrational spectrum of an important class of layered materials-the phosphorous chalcogenides of transition metals having the stoichiometric formula MPX 3, with M+Mn, Fe, Ni or Zn, and X=S or Se. They review our calculation of the dispersion relations and the phonon density of states preformed in the framework of an axially symmetric force-constant model. The same model is then employed to study the lattice dynamics of lithium-intercalated phases of iron chalcogenophosphates. In these calculations, the authors chose special values of the lithium concentration corresponding to commensurate lithium phases which keep the unit-cell size unchanged