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

[Image: see text] 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 La(10)(GeO(4))(6)O(3) and La(8)Bi(2)(GeO(4))(6)O(3) oxygen-excess materials, using the low-conductivity interstitial oxide-free La(8)Sr(2)(GeO(4))(6)O(2) as a benchmark. For La(10) and La(8)Bi(2), we locate the interstitial oxygen, O(int), responsible for conductivity by Rietveld refinement and relate the P6(3)/m to P1̅ phase transitions on cooling to oxygen ordering. Local structural studies using neutron total scattering reveal that well-ordered GeO(5) 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.