Reference : First-principles study of tungsten trioxide: Structural properties and polaron formation
Dissertations and theses : Doctoral thesis
Critical notes/edition
Physical, chemical, mathematical & earth Sciences : Physics
http://hdl.handle.net/2268/214218
First-principles study of tungsten trioxide: Structural properties and polaron formation
English
Hamdi, Hanen mailto [Université de Liège - ULg > > > Form. doct. sc. (physique - Bologne)]
1-Sep-2017
Université de Liège, ​Liège, ​​Belgique
Docteur en Sciences
Ghosez, Philippe mailto
[en] 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. 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 P2_1/c ground state. This non-polar P2_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 WO3 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 P21/c ground state, which makes WO3 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.

On the second hand, the addition of a single electron to the largest supercell of the monoclinic P21/c ground state of WO3 causes the development of a medium polaron inside of this material. We then study and characterize a medium polaron formation in WO3 from first-principles calculation. We show how the medium polaron in the supercell of WO3 can change its structural, electronic and dynamical properties.
http://hdl.handle.net/2268/214218

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