Mobile Polaronic States in α-MoO₃: An ab Initio Investigation of the Role of Oxygen Vacancies and Alkali Ions

Some oxides have the ability to trap excess electrons in the form of small polarons. Here, using first-principles techniques, we investigate the interaction of excess electrons with α-MoO₃. Polarons are found to be about 0.6 eV more stable than delocalized electrons. They can propagate with a high degree of anisotropicity along different crystallographic directions with the lowest barrier found to be about 0.08 eV. In addition to the band gap photoexcited charge carriers that can populate such polaron states, we investigate the role of oxygen vacancies as an intrinsic source of electrons. We also investigate intercalated alkali ions that can form complexes with the created polarons in the lattice. The alkali-polaron complex (A_xMoO₆, A = alkali ion) binding energies are relatively low, making it easy for the complex to dissociate. This, coupled with the low polaron migration energies, can generate a non-negligible contribution to electronic conductivity even in the absence of illumination, which is experimentally verified. Combined, this light-induced intercalation of alkali ion in MoO₃ and its subsequent deintercalation (complex dissociation) processes lead to a novel self-photocharghing phenomenon.