Lattice dynamics and hydrostatic-pressure-induced phase transitions in ScF3

Abstract

New phase transitions induced by hydrostatic pressure in a cubic (under standard conditions) ScF3 crystal are discovered by the methods of polarization microscopy and Raman scattering. The space groups $$R\bar 3c$$ for Z=2 and Pnma for Z=4 are proposed for the high-pressure phases. A nonempirical computation of the lattice dynamics of the crystal is carried out. It is shown that, under normal pressure, the cubic phase is stable down to T=0 K, while the application of a hydrostatic pressure gives rise to a phonon branch in the vibrational spectrum (between points R and M of the Brillouin zone) with negative values of squares of frequencies. The condensation of soft mode R5 at the boundary point of the Brillouin zone leads to rhombohedral distortion of the cubic structure with the unit cell volume doubling. The calculated frequencies at q=0 of the ScF3 lattice in the distorted rhombohedral phase are real-valued; the number and position of frequencies active in Raman scattering are in accord with the experimental values.