Structure and oxygen mobility in mayenite (Ca12Al14O33): a high-temperature neutron powder diffraction study

The structure of mayenite, Ca(12)Al(14)O(33), was investigated by neutron powder diffraction up to 1323 K. It has been described previously as a calcium-aluminate framework, in which 32 of the 33 oxygen anions are tightly bound, containing large cages, 1/6 of them being filled randomly by the remaining 'free' oxygen. At ambient temperature excess oxygen was found, corresponding to the composition Ca(12)Al(14)O(33.5) which was attributed to the presence of hydroxide, peroxide and superoxide radicals in the cages. Above 973 K these are lost under vacuum conditions and the composition becomes stoichiometric. From the refined structural parameters it is concluded that the structure is more adequately described as a relatively stable aluminate framework consisting of eightfold rings of AlO(4) tetrahedra with disordered Ca and 'free' O distributed within. At high temperatures the density of the 'free' oxygen is extremely spread out, with the expansion being related to the high ionic conductivity of this material. Since no continuous density distribution between adjacent cages was found and the 'free' O forms bonds with part of the Ca, the diffusion proceeds via a jump-like process involving exchange of the 'free' oxygen with framework oxygen. The results confirm the recent theoretical predictions of Sushko et al.