Fast divalent ion conduction—Ion ordering in β AND β″ alumina (Sr 2+, Cd 2+, Pb 2+)

Abstract

We have prepared single crystals of divalent ion rich β alumina and β″ alumina (Cd 2+, Sr 2+, Pb 2+) from sodium compounds by ion exchange. These compounds generally exhibit a similar composition (~0.83 M 2+ per unit in the conducting plane) and the same compensation mechanism for the non-stoichiometry (Mg 2+). X-ray studies (conventional structure determination and diffuse scattering) have been performed on these materials in order to establish the relationship between the structural and electrical characteristics. For the conducting ion organization, three typical cases have been displayed: a pure 2D local order when ions are localized in a quasi perfect conducting plane; a 3D local order when ions are localized in a conduction slab (ionic polarization of the host lattice); a 3D long-range order when ions are in-plane and out-plane on-centered from the theoretical position (ionic polarization of the host lattice and electronic polarization of the divalent ion).