Synthesis and characterization of the anionic conductor system La2Mo2O9-0.5xFx (x=0.02-0.30)

Fluorine-doped lanthanum molybdates with the nominal formula La2Mo2O9-0.5xFx (x = 0.20-0.30) were synthesized and characterized using powder X-ray diffraction, electron diffraction, differential thermal analysis, and impedance spectroscopy. Comparisons are made with the parent anionic conductor system La2Mo2O9. From our studies two distinct regimes with two different ranges of "x" values (range 1: including 0.00 less than or equal to x less than or equal to 0.12; range 2: including 0.18 less than or equal to x less than or equal to 0.30) could be delineated for the La2Mo2O9-0.5xFx compositions. Electron diffraction study on selected samples showed that x = 0.20 and 0.30 compositions (range 2) have a 3 x 3 x 3 supercell, different from the 2 x 3 x 4 supercell observed in the x = 0.00 and 0.10 compositions (range 1). All the fluorine-doped lanthanum molybdates show a first-order phase transition. With the increase in the fluorine content the transition temperature decreases in both ranges I and 2. Below the transition temperature, compositions belonging to range 2 have much lower conductivity when compared to that of those belonging to range 1. The activation energy as obtained from the conductivity data is higher for the fluorine-doped lanthanum molybdates compared to the parent compound La2Mo2O9. Above the transition temperature the conductivity of the fluorine-doped lanthanum. molybdate is higher than that of La2Mo2O9. Significantly our study presents the possibility of using fluorine substitution as a technique to increase the anionic conductivity in oxide ion conductors with favorable structural features (like in the case of La2Mo2O9) by decreasing the transition temperature of the first-order (order-disorder type) phase transition.