Structural, Electronic, and Thermodynamic Properties of the T and B Phases of Niobia: First-Principle Calculations.

Different polymorphs of NbO can be obtained depending on the pressure and temperature of calcination leading to different catalytic properties. Two polymorphs of niobia, T-NbO and B-NbO, have been investigated by means of density functional/plane waves method. The equation of state predicted that B-NbO phase is more stable than the T-NbO at low temperature; however at high pressure both phases are stable. These results are in good agreement with the available experimental data. The calculated cohesive energies of 6.63 and 6.59 eV·atom for the B-NbO and T-NbO, respectively, also corroborate this conclusion, and it can be compared to the experimental value of 9.56 eV atom estimated for the most thermodynamically stable phase. The topological analyses based on quantum theory of atoms in molecules (QTAIM) and electron localization function (ELF) were applied and reveal bonds with large ionic character for both phases. The B-NbO presented larger stiffness than T-NbO, and the oxygen sites in the T-NbO are more compressible. The density of states comparison for both structures indicates that B-NbO has lower concentration of acid sites compared to T-NbO. This result is consistent with the experimental observations that the concentration of Lewis acid sites decreases with the temperature.