We investigate using first-principles calculations the atomic structure of the orthorhombic phase of ${\mathrm{Ta}}_2{\mathrm{O}}_5$. Although this structure has been studied for decades, the correct structural model is controversial owing to the complication of structural disorder. We identify a new low-energy high-symmetry structural model, where all Ta and O atoms have the correct formal oxidation states of $+5$ and $-2$, respectively, and the experimentally reported triangular lattice symmetry of the Ta sublattice appears dynamically at finite temperatures. To understand the complex atomic structure of the ${\mathrm{Ta}}_2{\mathrm{O}}_3$ plane, a triangular graph-paper representation is devised and used alongside oxidation state analysis to reveal infinite variations of the low-energy structural model. The structural disorder of ${\mathrm{Ta}}_2{\mathrm{O}}_5$ observed in experiments is attributed to the intrinsic structural variations, and oxygen vacancies that drive the collective relaxation of the O sublattice.

• Received 18 February 2013

DOI:https://doi.org/10.1103/PhysRevLett.110.235502