Ab initio determination of Raman spectra of Mg2SiO4 and Ca2MgSi2O7 showing mixed modes related to LO/TO splitting

• Published in: Journal of Raman spectroscopy Vol. 52; no. 7; pp. 1346 - 1359
• Main Authors: Parlinski, Krzysztof, Piekarz, Przemysław
• Format: Journal Article
• Language: English
• Published: Bognor Regis Wiley Subscription Services, Inc 01.07.2021
• Subjects: ab initio calculations; Density functional theory; Dispersion curve analysis; Finite difference method; Forsterite; Light scattering; Luminous intensity; Mathematical analysis; mixed modes; Nonresonance; Phonons; Polarized light; Polycrystals; Raman spectra; Raman spectroscopy; Single crystals; Splitting; Tensors; åkermanite
• ISSN: 0377-0486; 1097-4555
• DOI: 10.1002/jrs.6129

Density functional theory was used to calculate the nonresonance Raman spectra of forsterite (Mg2SiO4) and åkermanite (Ca2MgSi2O7). 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 B2 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. The Raman spectra of two minerals are obtained from the ab initio calculations. Forsterite is one of the major mineral constituents of the Earth's crust and upper mantle and one of the most abundant silicates in the interstellar medium. In the åkermanite crystal, some infrared active modes have the same symmetry as the Raman active modes, and the LO/TO splitting is reflected in the mixed modes of the Raman spectra. ​