Mayenite (nominal composition Ca₁₂Al₁₄O₃₃), a major component of calcium aluminate cements, has recently attracted much attention for technological applications, e.g. as transparent conductive oxide, as catalyst for the combustion of volatile organic compounds or as a highly efficient oxygen ionic conductor. All this can be related to its particular crystal structure, which, in first approximation, consists of a calcium-aluminate framework, in which 32 of the 33 oxygen anions are bound. The remaining "free" oxygen is distributed at random over 1/6 of large cages in the framework and might diffuse through large openings between adjacent cages. More recent investigations [e.g. 1] have shown that the structure is heavily disordered involving e.g. displacements of Ca cations, and, at low temperatures, the presence of extra anion species like O^2-, O₂^2-, O^- and OH^-. The extra-framework oxygen can be exchanged by a variety of other anions. As a new approach we have successfully substituted it by nitrogen, therewith opening up possibilities for a first pure nitrogen ionic conductor.

Four samples have been synthesized via a solid-state route with different nitrogen contents of 0 (i.e. pure O-mayenite), 0.55, 1.0 and 1.27 wt-% N. Two iron doped samples with 0.1 and 2.5 mol-% Fe (with respect to Al content) were prepared via a sol-gel route. Neutron powder diffraction experiments were carried out at instrument SPODI (FRM2/Garching) using a wavelength of 1.548 Å and an Nb vacuum furnace up to 1050 °C. Synchrotron X-ray measurements were carried out at instrument B2 (Hasylab/Hamburg) with a wavelength of 0.49324 Å up to 900 °C using a graphite furnace. Data were analysed by the Rietveld method including anharmonic Debye-Waller factors using the program package JANA2000 and by difference Fourier methods. At ambient temperature the pure and N-doped samples contained extra anion species like peroxide, superoxide, hydroxide, imide and amide, which could partly be disentangled through the complementarities of the X-ray and neutron data. They disappear above ca. 700 °C whence the samples become stoichiometric. In contrast, the Fe doped samples are stoichiometric throughout. In particular, they do not contain any hydrogen making them more apt for technical applications. Analysis of the high temperature data with respect to the diffusion properties of these materials revealed e.g. that the diffusion of oxygen proceeds via a jump-like process involving exchange of "free" oxygen with framework oxygen, coupled to relaxations of the Ca ions [1]. In contrast, nitrogen diffuses as NH^2- via an interstitial process. Further details, also on irreversible changes, thermal expansion, etc. will be presented in this contribution.

This work was supported by the DFG within the priority program SPP 1136 under BO 1199/2 and LE 781/10.

[1] Boysen, H., Lerch, M., Stys, A., Senyshyn, A.: Acta Crystallographica (2007) B 63, 675. 

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