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

Oxides of tantalum (common examples including TaO, TaO₂, and Ta₂O₅) 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 TaO₃ via soft-chemical delamination of RbTaO₃. However, not much is known about the elusive nanosheet-structured TaO₃, unlike other closely related common trioxides of W and Mo. In this work, using first-principles density functional theory calculations, we have studied various TaO₃ structures as inspired from previous theoretical and experimental studies and discuss their properties with respect to the more conventional oxide of tantalum, Ta₂O₅. We have calculated their thermodynamics and lattice properties and have found a new stable-layered β-TaO₃ and its exfoliated monolayer phase (β′). By further analyzing their electronic structures, we discuss the mixed iono-covalent bonding characteristics in the TaO₃ phases, challenging the conventional formal oxidation state model for metal oxides. Finally, we propose how these new TaO₃ oxide materials may be potentially useful in photodevice applications.