The La_10 − x(SiO₄)₆O_{3 − 1.5x} (9.33 ≤ 10 − x ≤ 9.73) apatite series has been prepared and hexagonal single phases were obtained in a narrow compositional range (9.33 ≤ 10 − x ≤ 9.60). The room temperature crystal structure of La_9.55(SiO₄)₆O_2.32 has been determined from joint Rietveld refinement of neutron and laboratory X-ray powder diffraction data: a = 9.7257(1) Å, c = 7.1864(1) Å, V = 588.68(1) ų, Z = 1, R_wp ^N = 3.2%, R_wp ^X = 7.7%, R_F ^N = 1.8%, R_F ^X = 1.9%. An interstitial site for the extra-oxygen has been determined in the position very recently predicted in a theoretical study using atomistic simulations. The high temperature crystal structures have been obtained from neutron powder diffraction, NPD, collected at 773 and 1173 K showing the thermal evolution of this interstitial site. Previously reported neutron data for La_9.60(GeO₄)₆O_2.40 have also been re-analysed establishing the existence, and thermal evolution, of this interstitial site. The electrical results suggest that the samples are oxide ion conductors. The plots of the imaginary parts of the impedance, Z″, and the electric modulus, M″, vs. log (frequency), possess maxima for both curves separated by two decades in frequency. Bulk conductivities have been obtained from the fitting of the complex impedance spectra with the appropriate equivalent circuit. Bulk activation energies have been determined from two Arrhenius plots, one representing the bulk conductivities and the other representing the frequencies of the modulus peak maxima, f_max(M″). A comparative discussion of the two series, La_10 − x(TO₄)₆O_{3 − 1.5x} (T = Si, Ge), is given.

In this paper we report the successful incorporation of high valence transition metals, i.e. Cr, Mo, W, V, Nb, Ti, Zr into SrFeO3−δ perovskite materials, for potential applications as symmetric electrode materials for Solid Oxide Fuel Cells. It is observed that the doping leads to a change from an orthorhombic structure (with partial ordering of oxygen vacancies) to a cubic one (with the oxygen vacancies disordered). These electrodes are chemically compatibles with Ce0.9Gd0.1O1.95 (CGO) and La0.8Sr0.2Ga0.8Mg0.2O3−δ (LSGM) electrolytes at least up to 1100 °C. Thermal annealing experiments in 5% H2–Ar at 800 °C also show the stability of the doped samples in reducing conditions, suggesting that they may be suitable for both cathode and anode applications. In contrast, reduction of undoped SrFeO3−δ leads to the observation of extra peaks indicating the formation of the brownmillerite structure with the associated oxygen vacancy ordering. The performance of these electrodes was examined on dense electrolyte pellets of CGO and LSGM in air and 5% H2–Ar. In both atmospheres an improvement in the area specific resistances (ASR) values is observed for the doped samples with respect to the parent compound. Thus, the results show that high valence transition metals can be incorporated into SrFeO3−δ-based materials and can have a beneficial effect on the electrochemical performance, making them potentially suitable for use as cathode and anode materials in symmetrical SOFC