Interstitial oxygen conduction in lanthanum oxy-apatite electrolytes

Abstract

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.