Average and Local Structure of Apatite-Type Germanates and Implications for Oxide Ion Conductivity

Materials with the apatite structure have a range of important applications in which their function is influenced by details of their local structure. Here, we describe an average and local structural study to probe the origins of high-temperature oxide ion mobility in La₁₀(GeO₄)₆O₃ and La₈Bi₂(GeO₄)₆O₃ oxygen-excess materials, using the low-conductivity interstitial oxide-free La₈Sr₂(GeO₄)₆O₂ as a benchmark. For La₁₀ and La₈Bi₂, we locate the interstitial oxygen, O_int, responsible for conductivity by Rietveld refinement and relate the P6₃/m to P1̅ phase transitions on cooling to oxygen ordering. Local structural studies using neutron total scattering reveal that well-ordered GeO₅ square pyramidal groups form in the structure at low temperature, but that O_int becomes significantly more disordered in the high-conductivity, high-temperature structures, with a transition to more trigonal-bipyramid-like average geometry. We relate the higher conductivity of Bi materials to the presence of several O_int sites of similar energy in the structure, which correlates with its less-distorted low-temperature average structure.