Pressure-induced phase transitions in ScF3 crystal-Raman spectra and lattice dynamics

New pressure-induced phases are found in normally cubic ScF3 crystal using polarizing microscopy and micro-Raman spectroscopy. Their supposed space groups are R (3) over barc, Z = 2 and Pnma, Z = 4. Using ab initio model cubic phase was shown to be stable at ambient pressure down to T = 0 K, while hydrostatic pressure brings one of the phonon branches down to negative squared frequencies. R-5 soft mode condensation results in the rhombohedric distortion of the cubic lattice with cell volume doubling. Calculated squared frequencies of the high pressure phase are positive, their number and values agree with experimental results.