The Raman spectrum of diopside: a comparison between ab initio calculated and experimentally measured frequencies

Abstract

The Raman spectrum of diopside has been calculated by using three purely Density Functional Theory (DFT) Hamiltonians (PBE, WCPBE, LDA), the Hartree-Fock Hamiltonian (HF) and three hybrid HF/DFT ones (B3LYP, WC1LYP, PBE0). A comparison has been done between the calculated frequencies with those measured by Raman spectroscopy on a natural sample, along with several different orientations and beam polarizations, or retrieved from literature; such a comparison demonstrated the excellent performances of the hybrid Hamiltonians in reproducing the vibrational spectrum of the mineral, in line with what it is generally observed in literature concerning other mineral phases. In particular, the mean average absolute discrepancies of the calculated frequencies with respect to the experimental data were: 3.2 (WC1LYP), 4.7 (B3LYP), 6.5 (PBE0), 18.0 (PBE), 9.7 (WCPBE), 7.3 (LDA) and 40.6 cm -1 (HF). The very good quality of the WC1LYP results, allowed for a reliable assignment of all of the experimentally observed Raman signals, and the corresponding assignments to specific patterns of atomic vibrational motion (normal modes).

First Principles Thermodynamics of Minerals at HP–HT Conditions: MgO as a Prototypical Material

“…Minerals 2017, 7, 183 18 of 34 physical point of view as different minerals and compounds display a volume decrease with T not just in the low-P range [25,120], this phenomenon is systematically predicted by the HT-BM3-EOS at very HP-HT conditions. In particular, this usually happens beyond a threshold pressure which depends upon the thermophysical properties of the phase (and also on how these properties are defined), but it's typically around 25-30 GPa [63,113].…”

“…This has led different authors to develop alternative formulations (more or less empirical) to describe the P-V-T relations of minerals up to planetary interior conditions [4,63,78,[113][114][115][116][117][118][119]. Even though negative thermal expansion is obviously possible from a physical point of view as different minerals and compounds display a volume decrease with T not just in the low-P range [25,120], this phenomenon is systematically predicted by the HT-BM3-EOS at very HP-HT conditions. In particular, this usually happens beyond a threshold pressure which depends upon the thermophysical properties of the phase (and also on how these properties are defined), but it's typically around 25-30 GPa [63,113].…”

Density functional theory based calculations for high pressure research

“…on the description chosen to represent exchange-correlation effects, in essentially the same way as ground state structures and elastic properties discussed above. More accurate hybrid functionals such as B3LYP can reduce the mean-square error of calculated phonon frequencies by a factor of two, as has been shown in the case of diopside (Prencipe et al, 2012). Unfortunately such efforts are often based on a rather empirical approach that modifies parameters of a hybrid functional to achieve good agreement with experiment (e.g., Többens and Kahlenberg (2011)).…”

Hydrogen analysis in nominally anhydrous minerals by transmission Raman spectroscopy

“…The peak intensity and the area under the OHbands increased with increasing water content in the crystals. Low-wavenumber spectra measured with the 514 and 532 nm laser excitations corresponded to Raman spectra previously observed for diopside and grossular (e.g., Kolesov and Geiger 1998;Huang et al 2000;Arredondo and Rossman 2002;Prencipe et al 2012). The two dominant features for diopside are the bands at 667 and 1013 cm − 1 associated with a Si-O-Si bending mode and a Si-O stretching mode, respectively.…”