Thermal, structural and transport properties of the fast oxide-ion conductors La2−xRxMo2O9 (R=Nd, Gd, Y)

The thermal evolution of the crystal structure of the new fast oxide-ion conductor La2Mo2O9 across its structural α to β first order transition at 580 °C has been followed using synchrotron X-ray diffraction (XRD). The influence on the transition, structure and oxide-ion conductivity of partial substitution by other rare earths (Nd, Gd, Y) is studied. For Nd substitution the monoclinic α form is retained at room temperature in the whole compositional range. For Gd and Y substitutions, above a certain doping level, the suppression of the phase transition and stabilization at room temperature of the cubic β form are achieved. The cubic structures of La1.9Y0.1Mo2O9 at room temperature and of β-La2Mo2O9 at 670 °C (revisited), both refined from neutron powder diffraction patterns, are very similar. Despite the cell volume reduction, lanthanide substitution results above 500 °C in an increase of the anion conductivity compared to that of β-La2Mo2O9, which might result from differences in samplesʹ relative density.