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

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 (vertical bar Delta vertical bar) over bar 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