Properties of amorphous and crystalline titanium dioxide from first principles

Abstract

We used first-principles methods to generate amorphous TiO₂ (a-TiO₂) models and our simulations lead to chemically ordered amorphous networks. We analyzed the structural, electronic, and optical properties of the resulting structures and compared with crystalline phases. We propose that two peaks found in the Ti–Ti pair correlation correspond to edge-sharing and corner-sharing Ti–Ti pairs. Resulting coordination numbers for Ti (∼6) and O (∼3) and the corresponding angle distributions suggest that local structural features of bulk crystalline TiO₂ are retained in a-TiO₂. The electronic density of states and the inverse participation ratio reveal that highly localized tail states at the valence band edge are due to the displacement of O atoms from the plane containing three neighboring Ti atoms; whereas, the tail states at the conduction band edge are localized on over-coordinated Ti atoms. The $\Upgamma$-point electronic gap of ∼2.2 eV is comparable to calculated results for bulk crystalline TiO₂ despite the presence of topological disorder in the amorphous network. The calculated dielectric functions suggest that the amorphous phase of TiO₂ has isotropic optical properties in contrast to those of tetragonal rutile and anatase phases. The average static dielectric constant and the fundamental absorption edge for a-TiO₂ are comparable to those of the crystalline phases.