Energetics ofMnO2polymorphs in density functional theory

Abstract

We report the energetics and properties of the β, α, R, γ , λ, and δ polymorphs of MnO 2 within density functional theory, comparing the performance of the recently introduced SCAN functional with that of conventional exchange-correlation functionals and experiment. We find that SCAN uniquely yields accurate formation energies and properties across all MnO 2 polymorphs. We explain the superior performance of SCAN based on its satisfaction of all known constraints appropriate to a semilocal exchange-correlation functional and its accurate representation of all types of orbital overlap. DOI: 10.1103/PhysRevB.93.045132 First-principles thermodynamics has over the last decades matured into a reliable method for accessing the energetics of phase transitions and reactions in condensed matter systems. At the heart of this method lies Kohn-Sham density functional theory (DFT) [1], with its standard approximations to the exchange-correlation energy providing a reasonably accurate picture of electronic structure. One of the most basic results that can be derived from a set of DFT calculations is the ground-state structure of a given compound under some set of conditions, usually set as zero temperature and pressure. However, despite the importance of accurate first-principles structure determination for both materials and property analysis [2][3][4][5] this determination is often extremely difficult as the total energy differences between competing phases can be on the order of only a few meV per formula unit [6]. As a result, ground state structure selection is an attractive benchmark for verifying the adequacy of the physical model underlying a given approximation to the exchange-correlation energy.One particularly interesting system for investigating structure-transition energetics within DFT is the set of manganese oxides. The Mn-O system contains a diverse set of relatively well characterized structures both across a range of stoichiometries (MnO, Mn 3 O 4 , Mn 2 O 3 , and MnO 2 ), and within a single stoichiometry (pyrolusite β, ramsdellite R, hollandite α, intergrowth γ , spinel λ, layered δ MnO 2 ), as shown in Fig. 1. All the MnO 2 polymorphs share a common basic atomic structure-small Mn 4+ ions in a spin-polarized 3d 3 configuration and large, highly polarizable O 2-ions in a spin-unpolarized 2p 6 configuration, arranged in corner-and edge-sharing MnO 6 octahedra. The different packings of these octahedra form a variety of polymorphic structures, many of which have been studied extensively for applications in energy storage, catalysis, pigmentation, etc. [7][8][9][10][11][12][13][14][15]. * Corresponding author: gceder@berkeley.edu Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.The relative stability of the various oxidation states of Mn-O has been previously investigated within the PerdewBurke-Erzenhof (PBE) and...

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