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.
Low temperature crystal structures of apatite oxygen-conductors containing interstitial oxygen.
Oxygen-stoichiometric La(9.33) square(0.67)(Si(6)O(24))O2 and oxygen-excess La(8.65)Sr(1.35)(Ge(6)O(24))O(2.32) and La(8.65)Sr(1.35)(Si(6)O(24))O(2.32) oxy-apatites have been structurally characterized at low temperatures by the Rietveld method. Oxygen-interstitial distribution has been studied at 15 K for La(9.33) square(0.67)(Si(6)O(24))O2 and La(8.65)Sr(1.35)(Ge(6)O(24))O(2.32) by time-of-flight neutron powder diffraction and at 4 K for La(8.65)Sr(1.35)(Si(6)O(24))O(2.32) by constant-wavelength neutron powder diffraction. The low temperature structural study was undertaken in order to distinguish between the effects of static disorder, originated mainly from the presence of interstitial oxygens, and the anisotropic thermal vibrations. At such low temperatures, the influence of the anisotropic thermal vibrations is minimised. This structural study has firmly established the existence of interstitial oxygens in these materials, which may be useful as electrolytes for solid oxide fuel cells.
X-ray analysis of the incommensurate modulation in the 2:2:1:2 Bi-Sr-Ca-Cu-O superconductor including the oxygen atoms.
Single-crystal x-ray diffraction data on the 2:2:1:2 Bi-Sr-Ca-Cu-O superconducting phase have been used in a four-dimensional refinement of the modulations of both the metal and oxygen atoms. An expression for an analytical temperature-factor modulation function has been derived and applied to the bismuth atoms. The large value of the temperature factor of Bi in some of the unit cells indicates a contribution from static disorder. The modulation of the oxygen atoms in the bismuth layer is best described by a function that allows the displacement to vary linearly with unit-cell position and incorporates the possibility of excess oxygen near the end of a modulation period. The results on oxygen occupancy and the behavior of the Bi temperature factor are compatible with models that attribute the origin of the modulation to the existence of extra oxygen atoms in the Bi-O layers. Refinement in the noncentrosymmetric super space group {ital M}:{ital A}2{ital aa}:{bar 1}11 leads to significantly better results than the centrosymmetric treatment.