Structural stability, electronic and thermodynamic properties of VOPO4 polymorphs from DFT+U calculations

VOPO4 compounds are rich in polymorphs and have important applications in catalysis and as electronic functional materials. Tetragonal α1-, monoclinic α1, α2, and β polymorphs of VOPO4 have been investigated by using first principles calculations within density functional theory (DFT) improved by on-site Coulomb interactions (DFT+U) to better understand their thermodynamic and electronic properties. Structural and electronic properties, as well as relative stabilities for the four polymorphs, were calculated using DFT+U with both GGA and LDA exchange and correlation functionals. The effect of Hubbard U value was evaluated systematically and it is found that, although the relative stability of some polymorphs is not sensitive to the choice of U values, the relative stabilities between monoclinic α1 and α2 phases varied significantly with the choice of U values. The electronic density of states (EDOS) also changed with U values; the band gap first increased and then, when higher than 6, extra states were observed in the conduction band in some of the polymorphs, and the result was a band gap decrease. Clearly, an optimal U value in the range of 6-6.8 is desired for DFT+U calculations of VOPO4 polymorphs. Investigating the effect of change of U value on thermodynamic stability and electronic band gap behaviors can thus be a criterion to decide optimal U values for further calculations such as surface adsorption and surface reactions of these compounds.