Lattice dynamics of rhenium trioxide from the quasiharmonic approximation

Abstract

The quasiharmonic theory is applied to study the lattice dynamics and thermal properties of rhenium trioxide, a material exhibiting the negative thermal-expansion phenomenon. Phonons are calculated at several external pressures. The pressure dependence of the $M$, $R$, and zone-center phonon modes is analyzed. Relying on the Gr\uneisen formalism an influence of temperature on the $M$ phonon mode is investigated. The calculated free energy of the system provides predictions for the temperature dependence of such quantities as equilibrium volume and crystal thermal expansivity. The mean-squared vibrations of rhenium-trioxide atoms are investigated versus temperature and pressure. Results of the calculations indicate a softening of the $M$ and $R$ phonons with pressure and a large negative Gr\uneisen constants for these modes. A decrease in the crystal volume is observed up to 170 K. The thermal-expansion coefficient shows a minimum in the vicinity of 100 K which corresponds to a minimum of the anisotropy in the oxygen thermal vibrations perpendicular to the Re-O bond and along the bond. Anisotropy in the oxygen Debye-Waller factors follows from the anisotropy of the force constants at the oxygen site. Inelastic neutron-scattering measurements performed at room temperature are reported together with the generalized phonon densities of states. A comparison between present calculations and the experimental data as well as other theoretical studies is provided.