Lattice dynamics of layered MP$X_3$ (M=Mn,Fe,Ni,Zn; X=S,Se) compounds

An experimental and theoretical study of the lattice dynamics of MP$X_3$ layered compounds (M=Mn,Fe,Ni,Zn; X=S,Se) has been carried out. We present room-temperature infrared and Raman spectra and the phonon dispersion curves calculated in the framework of an axially symmetric force-constant model. The parameters of the dynamical model have been fitted on the Raman and ir data, consistently with the conditions of static equilibrium and of vanishing stress. A satisfactory assignment of the vibrational bands and a qualitative explanation of the dynamical differences among the materials are obtained. Moreover, the theoretical results support the hypothesis of phonon branches’ folding effects in the antiferromagnetic phase of ${\mathrm{FePS}}_3$ and provide the basis for an interpretive scheme of the vibrational spectra of lithium intercalated phases.