Unveiling defect physics in gapped metals: a theoretical investigation into defect formation and electronic structure interplay

In materials science, point defects play a crucial role in materials properties. This is particularly well known for the wide band gap insulators where the defect formation/compensation determines the equilibrium Fermi level and generally the doping response of a given material. Similarly, the main defect trends are also widely understood for regular metals (e.g., Cu and Zn discussed herein). With the development of electronic structure theory, a unique class of quantum materials – gapped metals (e.g., Ca₆Al₇O₁₆, SrNbO₃, In₁₅SnO₂₄, and CaN₂) that exhibit characteristics of both metals and insulators - has been identified. While these materials have internal band gaps similar to insulators, their Fermi level is within one of the main band edges, giving a large intrinsic free carrier concentration. Such unique electronic structures give rise to unusual defect physics, e.g., when the acceptor defect formation in n-type gapped metal results in the decay of the conducting electron to the acceptor states. In concentration limits, such electron-hole recombination can compensate for the energy needed to break chemical bonds and form acceptor vacancy, often leading to off-stoichiometric compounds. Such unusual physics, however, results make such quantum materials distinct from traditional compounds. Motivated by this, herein, we establish a minimal level of theory needed to account for the complex interplay between electronic structure and analyzing defects in gapped metals that can be utilized for their design in different practical applications.