| Abstract
| - 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, electrondiffraction, differential thermal analysis, and impedance spectroscopy. Comparisons are madewith the parent anionic conductor system La2Mo2O9. From our studies two distinct regimeswith two different ranges of “x” values (range 1: including 0.00 ≤ x ≤ 0.12; range 2: including0.18 ≤ x ≤ 0.30) could be delineated for the La2Mo2O9-0.5xFx compositions. Electron diffractionstudy on selected samples showed that x = 0.20 and 0.30 compositions (range 2) have a 3 ×3 × 3 supercell, different from the 2 × 3 × 4 supercell observed in the x = 0.00 and 0.10compositions (range 1). All the fluorine-doped lanthanum molybdates show a first-order phasetransition. With the increase in the fluorine content the transition temperature decreasesin both ranges 1 and 2. Below the transition temperature, compositions belonging to range2 have much lower conductivity when compared to that of those belonging to range 1. Theactivation energy as obtained from the conductivity data is higher for the fluorine-dopedlanthanum molybdates compared to the parent compound La2Mo2O9. Above the transitiontemperature the conductivity of the fluorine-doped lanthanum molybdate is higher thanthat of La2Mo2O9. Significantly our study presents the possibility of using fluorinesubstitution as a technique to increase the anionic conductivity in oxide ion conductors withfavorable structural features (like in the case of La2Mo2O9) by decreasing the transitiontemperature of the first-order (order−disorder type) phase transition.
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