Over-Stoichiometry in Heavy Metal Oxides: The Case of Iono-Covalent Tantalum Trioxides.

Oxides of tantalum (common examples including TaO, TaO, and TaO) are key oxide materials for modern electronic devices, such as dynamic random-access memory and field effect transistors. Of late, new forms of stable tantalum oxides have been proposed as two-dimensional nanosheet structures with a nonconventional stoichiometry of TaO via soft-chemical delamination of RbTaO. However, not much is known about the elusive nanosheet-structured TaO, unlike other closely related common trioxides of W and Mo. In this work, using first-principles density functional theory calculations, we have studied various TaO structures as inspired from previous theoretical and experimental studies and discuss their properties with respect to the more conventional oxide of tantalum, TaO. We have calculated their thermodynamics and lattice properties and have found a new stable-layered β-TaO and its exfoliated monolayer phase (β'). By further analyzing their electronic structures, we discuss the mixed iono-covalent bonding characteristics in the TaO phases, challenging the conventional formal oxidation state model for metal oxides. Finally, we propose how these new TaO oxide materials may be potentially useful in photodevice applications.