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 α-, β-, γ- and δ-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 α- and β-MnO2 were ductile, while γ-MnO2 was brittle. The phonon dispersion curves and the phonon density of states were calculated using DFPT. 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 Au, Au, B3u were IR characteristic band for α-, β-, γ-MnO2, and the symmetry of Ag, A1g, Ag, A1g were Raman characteristic band for α-, β-, γ- and δ-MnO2. Finally, the thermodynamic functions of the enthalpy change and the entropy were evaluated and compared with the experimental values.