Hidden Structural Order in Orthorhombic Tantalum Pentoxide (Ta$_2$O$_5$)

TL;DR

This study uses first-principles calculations to clarify the atomic structure of orthorhombic Ta$_2$O$_5$, revealing a new high-symmetry model and explaining experimental structural disorder through intrinsic variations and oxygen vacancies.

Contribution

It introduces a novel low-energy high-symmetry structural model for orthorhombic Ta$_2$O$_5$ and explains the observed disorder via structural variations and vacancies.

Findings

Identified a new high-symmetry structural model with correct oxidation states.

Demonstrated the dynamic appearance of triangular lattice symmetry at finite temperatures.

Attributed structural disorder to intrinsic variations and oxygen vacancies.

Abstract

We investigate using first-principles calculations the atomic structure of the orthorhombic phase of Ta$_2$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 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 Ta$_2$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 Ta$_2$O$_5$ observed in experiments is attributed to the intrinsic structural variations, and oxygen vacancies that drive collective relaxation of the O sublattice.