Aliovalent ionic substitution as a probe of conduction mechanism

Silver and sodium have qualitatively different diffraction-determined ionic distributions in the conduction planes of a β-alumina host. That this can imply different conduction mechanisms in the two cases is probed by partially exchanging Cd²⁺ ions into both compounds, and measuring the resulting mixed-ion distributions. The systems Ag_{1.22 − y}Cd_y/2Al₁₁O_17.11 (for y = 0, 0.10, 0.41 and 1.11) and Na_{1.22 − y}Cd_y/2Al₁₁O_17.11 for (y = 0, 0.22, 0.45 and 1.22) were investigated by single-crystal X-ray diffraction. The problematical use of diffraction techniques for highly disordered systems of this type is discussed; it is also shown how some of these difficulties can be reduced through access to additional physical and chemical information. Qualitatively different mechanisms are deduced in the two cases, suggesting that aliovalent substitution can indeed be a useful general tool in studying ionic conduction mechanisms in crystalline systems.