Theoretical study of and its high-pressure spinel phase

We perform a theoretical investigation of the (Formula presented) phase and the high-pressure spinel-like (Formula presented) phase of (Formula presented) The electronic structure is found to yield direct band gaps in the optical region for both phases. The vibration modes and their pressure dependence of (Formula presented) are determined theoretically, and are compared with experimental Raman spectra. All Raman-active modes are identified and agreement of theory with experiment is excellent. A Raman silent (Formula presented) mode of the (Formula presented) phase (Formula presented) is found to become soft under high pressure to yield a reduced symmetry (Formula presented)-phase derivative. The (Formula presented) structure changes further to a reduced symmetry (Formula presented) space group at higher pressure. Theory is used to determine the optimized structural parameters and equations of state (EOS) for all phases, and the EOS yields a theoretical value for the (Formula presented) phase-transition pressure. The vibration modes of the (Formula presented) phase are determined theoretically and compared to Raman measurements.