Ab initio quantum-mechanical prediction of the IR and Raman spectra of Ca3Cr2Si3O12 uvarovite garnet

The IR and Raman spectra of uvarovite (Ca3Cr2Si3O12) garnet were simulated with the periodic ab initio CRYSTAL code by adopting an all-electron Gaussian-type basis set and the B3LYP Hamiltonian. The two sets of 17 F1u Transverse-Optical (TO) and Longitudinal-Optical (LO) frequencies are generated, together with their intensities. As regards the IR experimental spectrum, only five peaks are available, that are in excellent agreement with the calculated data (mean absolute difference equation delta smaller than 5.2 cm−1). The analysis of the TO-LO eigenvalue overlaps permits to establish a correspondence between LO and TO modes. The set of experimental Raman peaks is much reacher (23 out of 25) and the agreement with our calculations excellent (delta smaller than 6 cm−1). Isotopic substitution is used to identify the zones of the spectrum where Cr and Ca contributions are relevant.