Study of the structural, vibrational and thermodynamic properties of natroxalate mineral using density functional theory

Highlights: • Natroxalate mineral was studied by solid-state density functional theory methods. • The structural properties and X-ray powder pattern were in very good agreement with experiment. • DFT calculations allowed to assign all observed Raman bands, including those at very low wavenumbers. • The bands at 1750 and 1358 cm^-1 correspond to an overtone and a combination band, respectively. • Fundamental thermodynamic properties of natroxalate were theoretically determined. - Abstract: Natroxalate mineral, Na₂C₂O₄, is a fundamental oxalate mineral widespread in nature, present in humans, animals and plants, as well as in naturally occurring minerals. The characterization of oxalate minerals is extraordinarily important since these organic minerals are indicators of environmental events and of the presence of biological activity, because they are commonly of biological origin. These minerals are currently under study to investigate the possible biological activity on Mars. The identification of these compounds is usually performed by X-ray diffraction and Raman spectroscopy. Theoretical calculations are of great value for the study and interpretation of the results of these experimental techniques. In this work, natroxalate mineral structure and Raman spectrum was studied by first principle calculations based on the density functional theory. The computed structure of natroxalate reproduces the one determined experimentally by X-ray diffraction (monoclinic symmetry, space group P2₁/c; lattice parameters a = 3.449 Å, b = 5.243 Å; c = 10.375 Å). Lattice parameters, bond lengths, bond angles and X-ray powder pattern were found to be in very good agreement with their experimental counterparts. Raman spectrum was then computed by means of density functional perturbation theory and compared with the experimental spectrum. Since the results were also found in agreement with the experimental data, a normal mode analysis of the theoretical spectra was carried out and used in order to assign the main bands of the Raman spectrum. The band found at about 567 cm^-1, described as a single peak in previous experimental works, is shown clearly to have two contributing bands. Finally, two bands of the observed spectrum, located at the wavenumbers 1750 and 1358 cm^-1, were not found in the theoretical spectrum. This is because these bands correspond to an overtone, 2ν₁ (ν₁ = 875 cm^-1), and a combination band, ν₁ + ν₂ (ν₁,ν₂ = 875, 481 cm^-1), respectively. Finally, the fundamental thermodynamic properties of natroxalate mineral were determined. The calculated specific heat at 298.15 K is in excellent agreement with the experimental value, the difference being less than 1%. Since for most of these properties there are not experimental values to compare with, their values were predicted.