Elaboração de um modelo baseado em IFCs para o cálculo da energia livre da fase alfa-F2

Abstract

Fluorine is the most reactive chemical element on the periodic table. Therefore, given the difficulty of conducting experiments at different pressures and temperatures, its phase diagram is poorly known. At ambient pressure and temperatures below 45 K, the most stable phase of fluorine is alpha-F2, a base-centered monoclinic crystal with space group C2/c. At higher pressures in the athermal limit, there is evidence of a phase transition to a base-centered orthorhombic structure with space group Cmce, which has only been explored through first-principles calculations. The difference in energy between these two phases is minimal, even lower than the accuracy of the first-principles methods. To calculate the free energy of the C2/c phase more accurately, this work aimed to calculate this quantity by incorporating non-harmonic contributions of the potential energy to the vibrational energy. Force constants were obtained by the direct method with the supercell approach. Different displacement-force data sets were explored using displacement patterns and random displacements. The determination of anharmonic phonons as a function of temperature was done by the self-consistent phonon theory. DFT calculations to obtain interatomic forces were carried out with the meta-GGA exchange-correlation functional r2SCAN with the non-local term rVV10, and with the hybrid PBE0 functional with Grimme correction. The vibrational frequencies obtained with r2SCAN+rVV10 were higher than those found by other researchers using first-principles and experimental methods, which indicates that this functional is not suitable for determining the vibrational frequencies of fluorine's intermolecular modes. With the PBE0+Grimme functional, we found harmonic frequencies consistent with the literature, although anharmonic spectra presented negative branches, indicating that further work is needed. [resumo fornecido pelo autor]