Structure and oxide anion conductivity in Ln2(TO4)O (Ln = La, Nd; T= Ge, Si)

Abstract Oxy-silicate and oxy-germanate, Ln 2 ( T O 4 )O ( Ln =La and Nd, T =Ge and Si) compounds have been prepared. Oxy-germanates can be readily obtained as highly crystalline single phases, while, the oxy-silicates are difficult to prepare as pure phases. The crystal structure of Nd 2 (SiO 4 )O has been studied from a joint Rietveld refinement of neutron and laboratory X-ray powder diffraction data. The electrochemical characterisation indicates that these compounds display oxide anion conductivity with p-type electronic contribution under oxidising conditions. The apparent activation energies under dry flowing nitrogen, where p-type contribution is minimised, are 0.97(1), 1.05(3) and 1.17(4) eV, for Nd 2 (SiO 4 )O, La 2 (GeO 4 )O and Nd 2 (GeO 4 )O, respectively. The overall conductivities at 1173 K range from 1.2×10 −4  S cm −1 for Nd 2 (SiO 4 )O to 1.3×10 −6  S cm −1 for La 2 (GeO 4 )O. Finally, the stability of these compounds under very reducing conditions has been studied and partial degradation is reported.