Electrical conductivity in oxygen-deficient phases of tantalum pentoxide from first-principles calculations

We apply first-principles density-functional theory (DFT) calculations, ab-initio molecular dynamics, and the Kubo-Greenwood formula to predict electrical conductivity in Ta2Ox (0 <= x <= 5) as a function of composition, phase, and temperature, where additional focus is given to various oxidation states of the O monovacancy (V-O(n); n = 0,1+,2+). In the crystalline phase, our DFT calculations suggest that V-O(0) prefers equatorial O sites, while V-O(1+) and V-O(2+) are energetically preferred in the O cap sites of TaO7 polyhedra. Our calculations of DC conductivity at 300 K agree well with experimental measurements taken on Ta2Ox thin films (0.18 <= x <= 4.72) and bulk Ta2O5 powder-sintered pellets, although simulation accuracy can be improved for the most insulating, stoichiometric compositions. Our conductivity calculations and further interrogation of the O-deficient Ta2O5 electronic structure provide further theoretical basis to substantiate V-O(0) as a donor dopant in Ta2O5. Furthermore, this dopant-like behavior is specific to the neutral case and not observed in either the 1+ or 2+ oxidation states, which suggests that reduction and oxidation reactions may effectively act as donor activation and deactivation mechanisms, respectively, for V-O(n) in Ta2O5. (C) 2013 AIP Publishing LLC.