Structures of four fast-ion conductors by single-crystal neutron diffraction analysis: Zn-stabilized Naβ''-alumina and Mg-stabilized Na-, K-, and Agβ''-aluminas

The crystal samples used were from the same batches as those used in other physical studies at Oak Ridge. Precise cell parameters were determined. The Zn2+ ion substitutes only in the Al(2) sites, as does Mg2+. The degree of substitution by Zn2+ (28.5% from chemical analysis and 29.3% from diffraction analysis) is less than that by Mg2+ (33.5% from chemical analysis and 34.9% for the average diffraction result from the three Mg-stabilized β''-aluminas). This implies more Na+ vacancies in Zn-stabilized Naβ''-alumina and may explain why it has a higher room temperature conductivity than Mg-stabilized β''-alumina. Fourier maps of the conducting-ion distributions are presented. The O(5) ions were treated as disordered in refinement of Mg-stabilized Ag- and Naβ''-aluminas. (Auth.)