A Drastic Influence of Point Defects on Phase Stability in MnO 2
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A Drastic Influence of Point Defects on Phase Stability in MnO2 Dane Morgan, Dinesh Balachandran, Gerbrand Ceder Department of Materials Science and Engineering, Massachusetts Institute of Technology Cambridge, Massachusetts 02139, USA Axel van de Walle Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 ABSTRACT Despite its importance as a cathode material in primary alkaline batteries, the structure of γ-MnO2 is still not well determined. Different authors have suggested that a number of different polymorphs, as well as highly disordered phases, may be present in γ-MnO2. The origin of this structural complexity remains largely unexplained. In this paper we use first principles methods to explore the energetics of the MnO2 system. We find a number of low-energy polymorphs with similar energies, suggesting that relatively small changes in the energetics might influence the stable phases. Using nonzerotemperature models we demonstrate that thermal disorder is not the cause of structural disorder in these materials. However, we then show that point (Ruetschi) defects, even in surprisingly low concentrations, have a dramatic effect on the phase stability. We propose that Ruetschi defects may be the key to some of the structural complexity in γMnO2, and that any realistic structural study must take them into account. INTRODUCTION The alkaline Zn/MnO2 system is the dominant chemistry used for primary batteries. The electrochemically active form of MnO2 presently used in alkaline batteries is commonly referred to as γ-MnO2. [1] It is often produced by electrolytic methods, in which case it is referred to as Electrolytic Manganese Dioxide (EMD). The γ-MnO2 polymorph does not denote a unique structure but has been suggested to be either a single phase with considerable disorder [1,2], or a multi-phase assembly, with some of the phases possibly being disordered [3,4]. However, all of the relevant phases of MnO2 that have been proposed consist of a (possibly distorted) hexagonal close packed lattice of oxygen ions with Mn cations occupying half the octahedral sites. Specific common Mn polymorphs have been given standard names: pyrolusite (β-MnO2, believed to be the most stable structure), ramsdellite (R-MnO2, often, in a defected form, believed to be the dominant component of γ-MnO2), and ε-MnO2 (generally described as a stochastic distribution of Mn on the octahedral sites of the oxygen framework) [5]. All polymorphs seen in nature (except perhaps ε-MnO2) have perfect Mn-Vacancy-Mn-Vacancy ordering along the c-axis of the hcp oxygen lattice. METHODOLOGY The energies of all configurations in this work are computed in the Generalized Gradient Approximation (GGA) to spin-polarized Density Functional Theory using ultra-
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soft pseudopotentials, as implemented in the Vienna Ab Initio Simulation Package (VASP). [6] Paramagnetic energies were calculated from a Heisenberg model fit to a number of collinear spin configurations. [7] The order-disorder temperature is obta
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