Ab initio simulations and experimental Raman spectra of Mg2SiO4 forsterite to simulate Mars surface environmental conditions

Kurzfassung

In this study we present the full Raman vibrational spectrum of forsterite (Mg2SiO4), computed from first principles, employing an hybrid HF/DFT Hamiltonian (WC1LYP) as implemented in the CRYSTAL14 code, at static equilibrium and at the temperatures of 0K, 300K and 1000K. The simulations are compared to the available literature data, confirming the accuracy of the calculations, and to experimental Raman spectra taken at room temperature on a natural sample of forsterite (Mg1.76 Fe0.22 SiO4), in order to test the effect of compositions on the reliability of a comparison between a computed spectrum for the endmember and the experimental spectrum on a real sample having a slightly different composition. The comparison with the experimental data at room temperature shows a very good agreement (an average discrepancy of 7 cm-1) and it allows a reliable symmetry assignment of Raman signals to specific vibrational modes. Spectra are also simulated by changing the mass of the nuclei for each of the six symmetry-independent species within the unit in order to quantify the contributions of each elements to the overall vibration. The aim is to relate the major experimental peaks, not only to specific structural features but also to link them to the Raman shifts observed due to both temperature and composition variation. Moreover, in order to foresee the possible response of Raman spectra to the different environmental conditions occurring on planetary surfaces, i.e. Mars, we calculate full Raman spectra at 0K, 300K and 1000K including zero point vibrational effects, within the limit of the quasi-harmonic approximation. These results may be useful to widen a Raman database and provide new clues to improve the interpretation of data acquisitions during the 2020 ExoMars mission, which will carry on board a Raman Laser Spectrometer (RLS).