First-principles re-investigation of bulk WO3

TL;DR

This study uses advanced first-principles calculations to analyze the structural phases of WO3, revealing that its low-temperature phase is likely non-ferroelectric and identifying potential ferroelectric and antiferroelectric phases.

Contribution

It provides a detailed first-principles re-investigation of WO3's structural phases, clarifying the nature of its ground state and identifying new ferroelectric phases.

Findings

The low-temperature phase of WO3 is non-intrinsically ferroelectric.

Identification of R3m and R3c ferroelectric phases with large polarizations.

Complex interplay of structural distortions affecting phase stability.

Abstract

Using first-principles calculations, we analyze the structural properties of tungsten trioxide WO3 . Our calculations rely on density functional theory and the use of the B1-WC hybrid functional, which provides very good agreement with experimental data. The hypothetical high-symmetry cubic reference structure combines several ferroelectric and antiferrodistortive (antipolar cation motions, rotations and tilts of oxygen octahedra) structural instabilities. The instability related to antipolar W motions combines with those associated to oxygen rotations and tilts to produce the biggest energy reduction, yielding a P21/c ground state. This non-polar P21/c phase is only different from the experimentally reported Pc ground state by the absence of a very tiny additional ferroelectric distortion. The calculations performed on a stoichiometric compound so suggest that the low temperature phase of WO3 is not intrinsically ferroelectric and that the reported ferroelectric character might arise from extrinsic defects such as oxygen vacancies. Independently, we also identify never observed R3m and R3c ferroelectric phases with large polarizations and low energies close to the P 21/c ground state, which makes WO3 a potential antiferroelectric material. The relative stability of various phases is discussed in terms of the couplings between different structural distortions, highlighting a very complex interplay involving improper-like couplings up to fourth order in the energy expansion in the cubic phase.