Structural studies of apatite-type oxide ion conductors doped with cobalt.
A series of Co doped lanthanum silicate apatite-type phases, La9.83Si4.5Co1.5O26, La9.66Si5CoO26, La10Si5CoO26.5 and La8BaCoSi6O26, have been synthesised, and neutron diffraction, EXAFS and XANES used to investigate their structures in detail. All compositions were shown to possess the hexagonal apatite structure, and the results confirmed that cobalt can be doped onto both the La and Si sites within the structure depending on the starting composition. The Co doping is shown to cause considerable local distortions within the apatite structure. In the case of Si site doping two compositions showed anisotropic peak broadening, which has been attributed to incommensurate ordering of oxygen within the apatite channels.
Oxide ion conductivity in mixed Si/Ge-based apatite-type systems
Abstract Apatite-type systems have been attracting significant attention recently due to their high oxide ion conductivity, offering the potential for use as the electrolyte in a solid oxide fuel cell (SOFC). In this article, we report the synthesis and conductivities of a range of novel mixed Si/Ge-based apatite systems, La 8.67 BaSi 6− y Ge y O 26 and La 9.33 Si 6− y Ge y O 26 (0≤ y ≤6). The results suggest that problems due to Ge volatility increase with increasing Ge content, but are partially reduced by Ba doping, which helps to stabilise the hexagonal apatite lattice. All the phases show high oxide ion conductivities, although the activation energy for oxide ion conduction shows a general increase with increasing Ge content. The mixed systems show the highest conductivities (e.g., σ 800 °C =0.06 S cm −1 for La 9.33 Si 2 Ge 4 O 26 ), and so are promising materials as electrolytes for SOFCs if problems due to Ge loss can be overcome.
Syntheses and superstructures of (Cd, M)5(PO4)3OH (M=Mn, Fe, Co, Ni, Cu, Hg)
Divalent ions of the first series transition elements and Hg were introduced into a cadmium hydroxyapatite Cd5(PO4)3OH by replacing a part of Cd2+, and the effects on the crystal growths and the structural changes were studied. Cd2+ can be exchanged with Mn2+ up to about 30 mol %, with Ni2+ or Hg2+ up to about 10 mol %, and with a little amount of Fe2+ or Cu2+. It was found that Cd5(PO4)3OH forms a superstructure when it contains a small amount of Mn2+, Fe2+, Co2+, Ni2+, Cu2+ or Hg2+ ion. Infrared spectra of Cd5(PO4)3OH containing these ions show some new bands in addition to those of Cd5(PO4)3OH. The average structures of the crystals containing a small amount of Hg2+ and Ni2+, and the superstructure of the crystal containing Mn2+ were refined with the full-matrix least-squares method, although the amounts and sites of the substituting ions could not be determined except for the crystals containing Hg2+. The superstructure belongs to P63 (a=16.1990(3), c=6.6485(1)A and Z=6) and has more distorted PO4 tetrahedra than the structure of pure Cd5(PO4)3OH.