Ab initio determination of Raman spectra of Mg ₂ SiO ₄ and Ca ₂ MgSi ₂ O ₇ showing mixed modes related to LO/TO splitting

Abstract Density functional theory was used to calculate the nonresonance Raman spectra of forsterite (Mg 2 SiO 4 ) and åkermanite (Ca 2 MgSi 2 O 7 ). The Raman tensors are determined from electric polarizibility using finite difference scheme. The Raman reduced spectra (wavenumbers and intensities) are calculated for several typical polarized light scattering geometries for single crystal and polycrystals, demonstrating good agreement with existing published data. In åkermanite, special attention is paid to B 2 and E symmetry phonon modes, which exhibit simultaneous infrared and Raman activities, labeled as mixed representations, and are involved in the longitudinal optic (LO)/transverse optic (TO) splitting. Close to zone‐center point, the phonon dispersion curves of mixed modes are analyzed in details, showing for example that mixed modes repulse each other.