Assignment of Raman-active vibrational modes of tetragonal mackinawite: Raman investigations and ab initio calculations

The mackinawite mineral was prepared as a carbon steel corrosion product in sulfidogenic waters at 90 degrees C after 2 months. The tetragonal crystal structure of the material was confirmed by Rietveld refinement of X-ray diffraction (XRD) data, and vibrational modes were analysed by micro-Raman spectroscopy. Despite a large number of studies on the formation and the stability of tetragonal mackinawite, the interpretation of the Raman spectra remains uncertain. In the present study, we report on the first calculation of the Raman-active vibrational modes of mackinawite using Density Functional Perturbation Theory and direct methods with BLYP + dispersion correction. Based on the comparison between calculated and experimental results, the four fundamental vibrational modes were assigned as 228 cm(-1) (B-1g), 246 cm(-1) (E-g), 373 cm(-1) (A(1g)) and 402 cm(-1) (E-g).