Quantum-mechanical ab initio simulation of the Raman and IR spectra of Mn₃Al₂Si₃O₁₂ spessartine

Abstract

The IR and Raman spectra of spessartine garnet Mn₃Al₂Si₃O₁₂, are simulated with the periodic ab initio CRYSTAL code by adopting an all-electron Gaussian-type basis set and the B3LYP Hamiltonian. The frequencies of the 25 Raman active modes (3 of A _1g , 8 of E _g and 14 of F _2g symmetry) and of the two sets of 17 F _1u transverse-optical and longitudinal-optical frequencies are generated, as well as the IR oscillator strength. The agreement between calculated and experimental data is excellent: for the IR and Raman sets, the mean absolute difference $\overline{|\Updelta|}$ is 4.0 and 6.8 cm⁻¹, respectively. Isotopic substitution permits to highlight the Mn, Al and Si participation to the various zones of the spectrum. Graphical animation, available on the authors’ web-site (http://www.crystal.unito.it/vibs/garnets/spessartine/), provides a very readable description of the movement of atoms and groups in each vibrational mode.