Application of simple lattice dynamics models to analysis of low-frequencyvibrational spectra of layered compounds

The frequencies of a few lattice and acoustic modes in trigonal KSc(MoO4)(2 ) have been calculated from the known velocities of longitudinal and transv erse acoustic waves propagating along the axis perpendicular to the layers. It has been shown that such calculations enable a precise assignment of so me lattice modes in KSc(MoO4)(2). The calculated frequencies have been furt her used to predict the respective frequencies for isomorphic compounds wit h K+ ions replaced by other alkali metal ions. Finally, these calculations may provide information on the origin of some low-frequency modes that appe ar as a result of the phase transition from the trigonal P3m1 to a monoclin ic or triclinic structure.