The Barrier to the Small Polaron Formation in Metal Peroxides

Polaron formation is the process by which free electrons (or holes) in a material find a lower energy localized state by distorting their surrounding host lattice. Since polaron formation involves changes in both electronic and atomic coordinates, a deeper understanding of polaron formation promises to open up new avenues towards tailoring the physical properties of materials. Ab initio studies have predominantly focused on studying stabilized small polaron states and their hoping physics, while less investigation has been devoted to the equally important process of polaron formation. In this work, we provide ab initio insights into the polaron formation process and focus on exploring the physical origins of the barrier to polaron formation. We utilize the HSE06 hybrid functional to study small polaron formation in four similar metal peroxides, which exhibit a wide range of polaron formation barriers heights. Our results show that polaron hybridization with the conduction band minimum (CBM) plays a significant role in determining the magnitude of a polaron formation barrier. Moreover, by satisfying the generalized form of Koopmans’ theorem, we are able to show that the degree of hybridization is directly correlated with the electronic relaxation delay during polaron formation.