Structures and properties of variously doped Mayenite investigated by neutron and synchrotron powder diffraction

Mayenite (Ca(12)Al(14)O(33)) is a promising new ionic conductor. This is related to its particular crystal structure, consisting of a calcium-aluminate framework, comprising 32 of the 33 oxygen anions. The remaining "free" oxygen is distributed over 1/6 of large cages in the framework and may diffuse through large openings between adjacent cages. The structure is heavily disordered involving displacements of Ca cations and the presence of extra anions like O(2)(-), O(2)(2-), O(-) and OH(-), which are released at high temperatures under vacuum conditions. The "free" oxygen has been substituted by nitrogen, opening up possibilities for a pure nitrogen ionic conductor. Material properties can be modified by doping on the cation sites. Detailed structure determinations with respect to disorder and ionic diffusion were carried out by neutron and X-ray synchrotron powder diffraction. At high temperatures diffusion of oxygen proceeds via a jump-like process involving exchange of "free" oxygen with framework oxygen, coupled to relaxations of the Ca ions. In contrast, nitrogen diffuses as NHS via an interstitial process. Fe doping leads to samples free of extra anions, larger disorder and promises higher ionic conductivities at high temperatures.