The anti-distortive polaron as an alternative mechanism for lattice-mediated charge trapping

Polarons can naturally form in materials from the interaction of extra charge carriers with the atomic lattice. Ubiquitous, they are central to various phenomena such as high-T_c superconductivity, electrochromism, photovoltaics, photocatalysis or ion batteries. However, polaron formation remains poorly understood and mostly relies on historical models such as Landau-Pekar, Fröhlich, Holstein or Jahn-Teller polarons. Here, from advanced first-principles calculations, we show that the formation of intriguing medium-sized polarons in WO₃ does not fit with traditional models but instead arises from the local undoing of distortive atomic motions inherent to the pristine phase, which lowers the bandgap through dynamical covalency effects and drives charge trapping. We introduce the concept of the anti-distortive polaron and rationalize it from a quantum-dot model. We demonstrate that anti-distortive polarons are generic to different families of compounds and clarify how this new concept opens concrete perspectives for a better control of the polaronic state and related properties.