SCHEDULED MAINTENANCE
Nitrogen and hydrogen defect equilibria in Ca12Al14O33: a combined experimental and computational study
Abstract
The defect structure of mayenite is investigated by Density Functional Theory (DFT) defect calculations; in situ electrical conductivity measurements in NH3 atmosphere at high temperature; and X-ray photoelectron spectroscopy (XPS) and gas phase mass spectrometry (GP-MS) of NH3 treated specimens. The computational results suggest that nitrogen is primarily incorporated substitutionally on oxygen sites as NH−2 and N3−. The concentration of nitrogen was estimated to be within the same order of magnitude by XPS, GP-MS and DFT, yielding a stoichiometry close to Ca12Al14O31.5N0.5:(NH2)0.5O0.5 which corresponds well with that obtained by Boysen et al. from similarly treated samples. Out diffusion of nitrogen was found to occur around 700 °C in Ar by XPS, GP-MS and conductivity measurements, also in accordance with Boysen et al. The conductivity measurements showed that NH3 treatment had a significant effect on the defect structure of the material which became evident only after replacing the NH3 atmosphere with Ar: the conductivity increased abruptly due to a temporary non-equilibrium reduction of the material as nitrogen diffuses out while the lack of a sufficiently large source of oxygen in the surrounding atmosphere prevents the specimen from re-oxidizing. Further, based on the computational results and the pH2 dependency on conductivity after NH3 treatment, we propose dissolution of hydride ions from H2 in the reduced and highly conductive post-NH3 state.
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