title: Structural, Electronic, and Thermodynamic Properties of the T and B Phases of Niobia: First-Principle Calculations.
abstract: Different polymorphs of Nb2O5can be obtained depending on the pressure and temperature of calcination leading to different catalytic properties. Two polymorphs of niobia, T-Nb2O5and B-Nb2O5, have been investigated by means of density functional/plane waves method. The equation of state predicted that B-Nb2O5phase is more stable than the T-Nb2O5at 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-1for the B-Nb2O5and T-Nb2O5, respectively, also corroborate this conclusion, and it can be compared to the experimental value of 9.56 eV atom-1estimated 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-Nb2O5presented larger stiffness than T-Nb2O5, and the oxygen sites in the T-Nb2O5are more compressible. The density of states comparison for both structures indicates that B-Nb2O5has lower concentration of acid sites compared to T-Nb2O5. This result is consistent with the experimental observations that the concentration of Lewis acid sites decreases with the temperature.
authors.affiliation: GPQIT, Departamento de Química, ICEx, Universidade Federal de Minas Gerais , Belo Horizonte 31270-901, MG, Brazil.
authors.forename: Mirele B
authors.initials: MB
authors.lastname: Pinto
authors.affiliation: GPQIT, Departamento de Química, ICEx, Universidade Federal de Minas Gerais , Belo Horizonte 31270-901, MG, Brazil.
authors.forename: Antonio Lenito
authors.initials: AL
authors.lastname: Soares
authors.affiliation: Departamento de Física, Facultad de Ciencias, Universidad de Chile , Santiago, Chile.
authors.forename: Andy
authors.initials: A
authors.lastname: Mella Orellana
authors.affiliation: GPQIT, Departamento de Química, ICEx, Universidade Federal de Minas Gerais , Belo Horizonte 31270-901, MG, Brazil.
authors.forename: Hélio A
authors.initials: HA
authors.lastname: Duarte
authors.affiliation: GPQIT, Departamento de Química, ICEx, Universidade Federal de Minas Gerais , Belo Horizonte 31270-901, MG, Brazil.
authors.forename: Heitor A
authors.identifier: http://orcid.org/0000-0001-5324-6010
authors.initials: HA
authors.lastname: De Abreu