Raman modes in Pbca enstatite (Mg2Si2O6): An assignment by quantum mechanical calculation to interpret experimental results

Raman spectra of orthoenstatite have been computed from first principles, employing the hybrid HamiltonianWC1LYP.[1] The calculated data show excellent agreement with the experimental data from literature with an absolute average difference of ~5 cm^-1. The quantum mechanical simulation allowed the assignment of Raman features to specific vibrational modes. This enabled to assess quantitatively the contributions of internal (tetrahedral stretching) and external (tetrahedral chains and M1 and M2 cations) vibrations. Moreover, the mass substitution of 56-Fe for 24-Mg in the M1 and M2 sites and of 30-Si and 18-O for the 28-Si and 16-O sites, pointed out the relative contributions of the cations to each mode within different sites. The description of the Raman modes enabled to relate themajor experimental peaks to specific structural vibrations, and to link the changes in crystal structure to those modes with pressure, temperature and composition. The results provide new clues to identifymost suitable peaks for the investigation of the intracrystalline ordering of Fe and Mg in the M1 and M2 sites, and of Al in the tetrahedral and octahedral sites. Moreover we have been able to identify those peaks which are related to structural features, like tetrahedral bond distances.