First-principles reinvestigation of bulk WO3

Using first-principles calculations, we analyze the structural properties of tungsten trioxide ${\mathrm{WO}}_{3}$. Our calculations rely on density functional theory and the use of the B1-WC hybrid functional, which provides very good agreement with experimental data. We show that the hypothetical high-symmetry cubic reference structure combines several ferroelectric and antiferrodistortive (antipolar cation motions, rotations, and tilts of oxygen octahedra) structural instabilities. Although the ferroelectric instability is the largest, the instability related to antipolar W motions combines with those associated to oxygen rotations and tilts to produce the biggest energy reduction, yielding a $P{2}_{1}/c$ ground state. This nonpolar $P{2}_{1}/c$ 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 ${\mathrm{WO}}_{3}$ is not intrinsically ferroelectric and that the experimentally observed ferroelectric character might arise from extrinsic defects such as oxygen vacancies. Independently, we also identify never observed $R3m$ and $R3c$ ferroelectric metastable phases with large polarizations and low energies close to the $P{2}_{1}/c$ ground state, which makes ${\mathrm{WO}}_{3}$ a potential antiferroelectric material. The relative stability of various phases is discussed in terms of the anharmonic couplings between different structural distortions, highlighting a very complex interplay.