First principles calculation of mechanical, dynamical and thermodynamic properties of MnO2 with four crystal phases

MnO2 has four crystal phases, whose stability and vibrational properties can affect significantly its application. In this study, a spin-polarized DFT + U method was performed to investigate the structural, mechanical, lattice dynamical, and thermodynamic properties of alpha-, beta-, gamma- and delta-MnO2. The analysis of the mechanical properties and phonon dispersion curves confirmed that all four kinds of MnO2 were mechanically and dynamically stable. The calculated mechanical properties indicated that alpha- and beta-MnO2 were ductile, while gamma-MnO2 was brittle. The phonon dispersion curves and the phonon density of states were calculated using DEPT. The phonon vibrational frequencies were analyzed in detail by the LO-TO splitting considering the non-analytical term correction. Raman and infrared active normal modes were assigned and discussed briefly to resolve the uncertainty in experiment of the assignment for the four kinds of MnO2. The symmetry of A(u), A(u), B-3u were IR characteristic band for alpha-, beta-, gamma-MnO2, and the symmetry of A(g), A(1g), A(g), A(1g) were Raman characteristic band for alpha-, beta-, gamma- and delta-MnO2. Finally, the thermodynamic functions of the enthalpy change and the entropy were evaluated and compared with the experimental values. (C) 2020 Elsevier B.V. All rights reserved.