Lattice vibrations in high-pressure phases of LiYF4

Possible variations in the dynamical behaviour of LiYF4 due to its several structural changes under pressure are examined by quasi-harmonic lattice dynamics calculations. The phonon spectra in the entire Brillouin zone together with the respective Gibbs free energies are calculated for the three high-pressure polymorphs of LiYF4 (that are stable at T=0) with a view to better understand their relative stabilities as functions of pressure and temperature. The present work predicts anomalous thermal expansion at low temperatures in phases I and IIa. Molecular dynamics simulations provide qualitative impressions about a temperature-driven second-order transition and also of kinetic effects in the subsequent pressure-driven first-order phase transformation.