Invited
Origin of polarons in WO₃ and related perovskites

Polarons are quasi-particles resulting from the interactions of extra charge carriers (electrons or holes) with the atoms of the crystal lattice. They can be seen as a composite object formed by the charge itself and a cloud of virtual phonons surrounding it. Conceptualized by Landau in 1933, the notion of polaron was further rationalized by Pekar in 1946 and gave rise over the years to numerous theories and experimental observations. Nowadays, polarons are accessible to first-principles simulations and second-principles methods also open exciting new perspectives for modeling them at finite temperature.

ABO₃ perovskites, including prototypical ferroelectrics like BaTiO₃, are highly polarizable media prone to develop polarons that can further strongly affect their properties. In this talk, I will first provide a short overview of recent advances in the first-principles modeling of polarons in ABO₃ oxide perovskites. Then, taking the prototypical example of WO₃, I will illustrate that such calculations are not only powerful to reproduce polarons but also particularly insightful to unravel their physical origin. Amazingly, our calculations indeed reveal that, when an extra electron is added into ground-state phase of WO₃, it dominantly activates non-polar rather than polar phonon modes. This observation implies that, in addition to conventional Fröhlich’s view, there should be another mechanism to stabilize the charge. A simple model will be presented that unveil and rationalize how this is intimately linked to changes of hybridization between W 5d and O 2p orbitals. The relevance of this new model to related ABO₃ perovskites will be discussed.

Work done in collaboration with H. Hassani, B. Partoens, J. Chen and E. Bousquet and supported by F.R.S.-FNRS Belgium (PDR project PROMOSPAN) and the European Union’s Horizon 2020 research and innovation program under grant agreement N° 766726 (TSAR).