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Understanding Oxygen Nonstoichiometry in Mayenite: From Electride to Oxygen Radical Clathrate

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journal contribution
posted on 16.04.2019, 00:00 by Zheng Yu, Monday Uchenna Okoronkwo, Gaurav Sant, Scott T. Misture, Bu Wang
Mayenite (Ca12Al14O32+δ) is a unique material that forms an electride stable at room conditions when reduced and shows oxygen storage capability when oxidized. The distinct redox behavior and properties of mayenite are known to originate from its clathrate structure that can host a range of oxygen species as well as anionic electrons. However, the oxygen nonstoichiometry in mayenite remains poorly understood. In this work, we establish, based on ab initio simulations, a systematic understanding of the oxygen nonstoichiometry in mayenite in relation to redox conditions and the speciation of the clathrated oxygen ions. It is found that the nonstoichiometric mayenite can be divided into three regions: (i) electride region (0 < δ < 1), wherein the energetically preferred clathrated species are O2– and anionic electrons, (ii) oxidized region (1 < δ < 4), wherein a mixture of O, O2–, and O2 ions can be clathrated in the nanocages, and (iii) strongly oxidized region (4 < δ < 12), wherein the O2-clathrate form is the most stable. The computed redox energetics suggests that the stability of the mayenite electride at room conditions is due to a kinetically limited oxidation process and achieving superstoichiometry is thermodynamically possible in conditions such as in air between 300 and 1000 K. On the basis of the simulation results, the Ellingham diagram for the mayenite electride is computed by considering the different anionic electron concentrations and the chemical potentials for various oxygen species are established to facilitate the development of mayenite as a versatile functional material.