Theoretical study of β-Ge3N4 and its high-pressure spinel γ phase

nWe perform a theoretical investigation of the beta phase and the high-pressure spinel-like gamma phase of Ge3N4. 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 beta-Ge3N4 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 B-u mode of the beta phase (P6(3)/m) is found to become soft under high pressure to yield a reduced symmetry (pg) P-phase derivative. The P structure changes further to a reduced symmetry P3 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 beta-->gamma phase-transition pressure. The vibration modes of the gamma phase are determined theoretically and compared to Raman measurements.