Average and Local Structure of Apatite-Type Germanates and Implications for Oxide Ion Conductivity
Average and local structure of apatite-type germanates and implications for oxide ion conductivity.', The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be
... l-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full DRO policy for further details. ABSTRACT: 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 R = La and Bi we locate the interstitial oxygen, Oint, responsible for conductivity by Rietveld refinement and relate the P63/m to P1 � phase transitions these phases undergo 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 Oint become 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 for R = Bi to the presence of several Oint sites of similar energy in the structure, which correlates with its less-distorted low temperature average structure.