Electronic and magnetic properties of α-MnO2 from ab initio calculations

alpha-MnO2, an active catalyst for oxygen reduction and evolution reactions, has been investigated using ab initio calculations with different exchange-correlation functionals: the generalized-gradient approximation in the version of Perdew, Burke, and Ernzerhof (PBE), PBE + U, and hybrid functionals. Both hybrid functionals and PBE + U (U >= 2.0 eV) fail to capture the antiferromagnetic (AFM) ground state found experimentally, and a ferromagnetic configuration has the lowest energy. An AFM ground state is then recovered when using PBE or PBE + U (U <= 1.6 eV). Interestingly, a reduction of the gap is observed at increasing values of the U parameter. We offer a qualitative explanation for the change in the calculated ground state employing the results for the electronic structure and physical arguments similar to those exposed in the Goodenough-Kanamori-Anderson rules. It is argued that the p(z) orbital of oxygen atoms with sp(2) hybridization plays a fundamental role in the superexchange AFM interaction and in the reduction of the gap.