Vibrational studies on up to 40 GPa by Raman spectroscopy at room temperature

Raman spectra of (Formula presented) were obtained under hydrostatic (in (Formula presented)) and nonhydrostatic (pure (Formula presented)) pressures up to 40 GPa at room temperature. The pressure-induced changes of the librational spectra suggest that solid (Formula presented) does not transform directly from phase I (cubic (Formula presented)) to phase III (orthorhombic (Formula presented)). An intermediate phase IV, which may involve only minor modifications of phase I, is formed prior to the transition to (Formula presented) structure. Phases IV and III coexist over some pressure range. The pressure shifts of all four Raman-active librons of phase III were determined over a wide pressure range and are compared to previous theoretical results. Splittings in the (Formula presented) and (Formula presented) modes are observed in the stability field of phase I below 10 GPa. The frequency of the (Formula presented) mode decreased slightly whereas the frequency of the (Formula presented) mode increased slightly at the transition to phase III. The liquid-to-solid transition at 0.6 GPa was confirmed.