First-principles study of the structure of stoichiometric and Mn-deficient MnO2

We present an extensive Density Functional Theory study on the phases and magnetic states of MnO2, with over 300 calculations of various Mn-vacancy configurations and magnetic spin states. It is shown that the paramagnetic extrapolations of spin-polarized results are essential to correctly reproduce pyrolusite as the ground state of MnO2. Paramagnetic energies are obtained by fitting a Heisenberg Hamiltonian to the energy of 10-20 magnetic configurations for each of 16 possible MnO2 polymorphs. Near groundstate degeneracy is shown to occur due to the frustration of otherwise large interactions. While many other structures are found to be near degenerate in energy with pyrolusite, no thermal disorder exists in the system up to several thousand degrees. The thermal disorder is suppressed because the strong correlation of the Mn-vacancy order along the lines of face-sharing octahedra removes any low-energy excitations from the system. Mn vacancies compensated by protons (Ruetschi defects), ubiquitously present in commercial MnO2, are shown to have a dramatic effect on phase stability. The stabilizing effects of Ruetschi defects may explain the presence in MnO2 of ramsdellite and twinning, both of which are unstable in the pure material. We believe Ruetschi defects to be an important source of the structural complexity of synthetic MnO2 produced either electrochemically or chemically. (C) 2003 Elsevier Science (USA). All rights reserved.