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 La10 and La8Bi2, we locate the interstitial oxygen, Oint, 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 Oint 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 Oint sites of similar energy in the structure, which correlates with its less-distorted low-temperature average structure.
Highlights
Apatites are materials with the general formula A10(TO4)6X2±x, where A = alkaline or rare earth metal; M = Ge, Si, or P; and X = halides, O2−, or [OH] −
Symmetrylowering deviations from the ideal apatite structure and differing arrangements of ions in its channels are important to the function of natural bone and tooth enamel and the use of apatite-type synthetic materials in bone grafts and implants.[1−5] In particular, the details of the local coordination environment of Ca2+ ions are necessary for understanding bone formation and diseases, but this information is hard to obtain given the experimental difficulty of techniques such as 43Ca solid-state NMR.[6−8] Similar research questions regarding apatite-type materials are important in archeological science and anthropology.[9−12]
The local environment and symmetry of the crystallographic sites occupied by the dopant activator ions significantly influence luminescence probabilities and energy transfer processes, and the emission properties of the phosphors.[15−18] Certain apatite-type lanthanum germanates and silicates exhibit high oxide ion conductivities, making them applicable in oxygen sensors and pumps,[19] separation membranes,[20,21] and solid oxide fuel cells (SOFCs).[22−24] These applications are the focus of this paper
Summary
Apatites are materials with the general formula A10(TO4)6X2±x, where A = alkaline or rare earth metal; M = Ge, Si, or P; and X = halides, O2−, or [OH] − Compounds adopting this structure type are ubiquitous both as natural biomaterials and as synthetic functional materials with a range of technological applications. In the context of modern technological applications, departures from the ideal centrosymmetric crystal structures make some apatite-type materials second harmonic generation (SHG) active and potentially suitable for nonlinear optical (NLO) applications.[13,14] Apatite-type oxides are promising hosts for the development of phosphors for solid-state lighting In this application, the local environment and symmetry of the crystallographic sites occupied by the dopant activator ions significantly influence luminescence probabilities and energy transfer processes, and the emission properties of the phosphors.[15−18] Certain apatite-type lanthanum germanates and silicates exhibit high oxide ion conductivities, making them applicable in oxygen sensors and pumps,[19] separation membranes,[20,21] and solid oxide fuel cells (SOFCs).[22−24] These applications are the focus of this paper
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