Raman modes inPbcaenstatite (Mg₂Si₂O₆): an assignment by quantum mechanical calculation to interpret experimental results

Abstract

Raman spectra of orthoenstatite have been computed from first principles, employing the hybrid Hamiltonian WC1LYP. 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 56Fe for 24Mg in the M1 and M2 sites and of 30Si and 18O for the 28Si and 16O sites, pointed out the relative contributions of the cations to each mode within different sites. The description of the Raman modes enabled to relate the major 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 identify most 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. Copyright © 2016 John Wiley & Sons, Ltd.

High‐pressure Raman spectroscopy of Ca(Mg,Co)Si2O6 and Ca(Mg,Co)Ge2O6 clinopyroxenes

“…The high effect on the Raman peak positions of small changes in interatomic distances in tetrahedra, hardly detected in the structure refinements based on high‐pressure single‐crystal X‐ray diffraction data, was observed also in the orthopyroxene enstatite …”

“…The high effect on the Raman peak positions of small changes in interatomic distances in tetrahedra, hardly detected in the structure refinements based on high-pressure single-crystal X-ray diffraction data, was observed also in the orthopyroxene enstatite. [43] The same is found for pyroxenes of the series CaMgSi 2 O 6 ─CaCoSi 2 O 6 , where the structural changes in the tetrahedron are negligible by single-crystal X-ray diffraction, [45] and the tetrahedral bond lengths appear to be unchanged in all the series. The strong changes in higher wavenumber modes have been related only to small differences in the tetrahedral distortion.…”

“…Factor-group analysis at Г point (k = 0) shows that C2/c pyroxenes, with 20 atoms in the reduced primitive unit cell (Z = 2), have 30 (14 A g + 16 B g ) Raman active modes. [26,42] Recent quantum mechanical calculation of the vibrational modes in diopside and orthoenstatite [27,43] showed that the vibrational modes are a complex mixture of bending and stretching-like vibrations involving different atoms: In general, almost pure stretching modes may be found only at wavenumbers higher than 1000 cm À1 . In terms of prevailing mode vibration, in the wavenumber range below 500 cm À1 , modes associated with cation translations as well as longer-wavelength lattice modes are found; the different response of the vibrational modes to changes in pressure, temperature and composition depends on whether they are involved in structural changes of the M1 or M2 polyhedra.…”

Recent advances in linear and non‐linear Raman spectroscopy. Part XI

“…Stangarone and co‐workers studied the Raman modes in Pbca enstatite (Mg 2 Si 2 O 6 ) and gave an assignment based on quantum mechanical calculations to interpret experimental results. The calculated data show excellent agreement with the experimental data from literature with an absolute average difference of ∼5 cm −1 …”

“…The calculated data show excellent agreement with the experimental data from literature with an absolute average difference of ∼5 cm −1 . [146]…”