Disorder and Diffusion in Mayenite

Abstract

Mayenite, Ca12Al14O33, has attracted enormous attention for novel technological applications after the discovery of its high oxygen ionic conductivity. The crystal structure consists of a calcium-aluminate framework, comprising 32 of the 33 oxygen anions. The remaining oxygen is distributed over 1/6 of large cages within the framework. The true structure is heavily disordered and usually non-stoichiometric due to the presence of extra anions and is presented for four samples: pure oxygen mayenite (O-mayenite), partly (O/N-mayenite) and fully (N-mayenite) exchanged by nitrogen and doped with iron (Fe-mayenite). All samples were investigated by neutron powder diffraction up to 1050 C. Data were analysed by the Rietveld method and by difference Fourier methods. As prepared O-mayenite contains O - 2 and OH -, N-and O/N-mayenite also NH - 2 , NH 2-and N 3-, while Fe-mayenite was free of extra radicals. In O-and N-mayenite the extra species are lost under vacuum conditions above ca. 700 C, whereas O/N-mayenite retained NH - 2 up to high temperatures. Diffusion of oxygen proceeds via a jump-like interstitialcy process involving exchange of the "free" oxygen with framework oxygen, coupled to relaxations of Ca ions. In O/N-mayenite NH - 2 diffuses via interstitial process. In Fe-mayenite encaged oxygen is "invisible" due to extreme delocalisation or loss.