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

Abstract

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.

Formation and Transport Mechanisms of Hydrogenous Species in Mayenite

“…Note that the variation of the chemical potentials due to different configurations of occupied cages is small (e.g., <0.15 eV in the case of clathrated O 2– ). This is expected based on our previous study, which suggests that the interaction between the clathrated species occupying different cages does exist through cage deformation but is weak . We confirm this by calculating the formation enthalpy of eq via explicitly simulating structures containing the same species (Table , direct simulations), which is found to be in good agreement with those estimated by using the chemical potentials.…”

“…We first investigate the energetics associated with incorporating different hydrogenous species in mayenite. We follow our previous study and report the energetics as the chemical potential for each species, i.e., the energy difference of mayenite structures containing these species with respect to electride (C12A7:2e – ) . On the basis of these chemical potentials (Table ), we estimate the thermodynamic stability of mayenite containing different combinations of clathrated hydrogenous species.…”

“…We follow our previous study and report the energetics as the chemical potential for each species, i.e., the energy difference of mayenite structures containing these species with respect to electride (C12A7:2e − ). 23 On the basis of these chemical potentials (Table 1), we estimate the thermodynamic stability of mayenite containing different combinations of clathrated hydrogenous species. From here, different reactions that may occur in mayenite upon H 2 treatment are compared, and the results are tabulated in Table 2.…”

“…Based on our previous band structure calculations, the activation energy for anionic electronic hopping is within the range of 0.4 to ∼1 eV (depending on the electron concentration). 23 This may explain the reduced activation energy observed experimentally.…”

Localization Mechanism of Interstitial Electronic States in Electride Mayenite

“…At the same time in another work 21 it was claimed that maximum entropy method (MEM)-Rietveld combined analyses of synchrotron powder X-ray diffraction patterns together with DFT calculations indicate that the electrons are delocalized over the cages. Furthermore, the results of the DFT 4,28,30,31 demonstrate a negative slope of the density of states (DOS) curve at the Fermi level, which indicates a negative sign of the Seebeck coefficient (charge carrier sign), while in the experiment 20,24 it is positive for the metallic phase of [Ca 12 Al 14 O 32 ] 2+ (e − ) 2 . Furthermore, using high-resolution photoemission spectroscopy, a small but welldefined peak at the Fermi level (E F ) was clearly observed, 31 while DFT 4,28,30,31 does not show any tendency to form such a peak.…”