Ionic conductivities and phase transitions of lanthanide rare-earth substituted La2Mo2O9

The ion conductivities and phase transitions of lanthanum molybdate (La2Mo2O9) substituted with lanthanide rare-earths are investigated using impedance spectroscopy, dilatometry, and X-ray powder diffraction. Among the substituted La2Mo2O9 of 10 mol% Ce, Nd, Sm, Gd, Dy, Er, Yb, the specimens containing Er, and Dy exhibit depressed α-β phase transformation and high conductivities. Their 700 °C conductivities are approximately five to seven times that of La2Mo2O9, around 0.26 S cm−1, comparable with those of (LaSr)(GaMg)O3 and Gd-substituted CeO2. Among the three compositions of 10 mol% Gd, Dy, Er showing depressed phase transition, Er- and Dy-substituted La2Mo2O9 possess relatively low thermal expansion coefficient 11×10−6 K−1, compared with that of the Gd-substituted La2Mo2O9, 18×10−6 K−1, which is near that of La2Mo2O9. Hence, Dy and Er are valuable dopants in improving the La2Mo2O9 properties. Across the lanthanide series, 10 mol%-substituted La2Mo2O9 demonstrates systematic variations in the conductivity-temperature relation. Hysteresis phenomena in both of conductivity and thermal expansion are also observed in those compositions which display phase transition.