Phase formation, structural features and electrical properties of praseodymium molybdate Pr4MoO9+δ synthesized by solid-state and Sol–Gel routes

Abstract

The paper describes the phase formation, structure and electrical properties of praseodymium molybdate Pr4MoO9+δ synthesized by solid-state and citrate sol–gel routes. Simultaneous thermal analysis (DSC/TG) combined with X-ray diffraction shows that the Pr6O11 reacts with MoO3 proceeds via Pr2(MoO4)3 and Pr2MoO6 intermediates; the target monoclinic phase (space group C 2/m) appears at ∼1100 °C, while in air it decomposes in an entropy-driven manner at ∼1200 °C into a fluorite-type PrO2−δ solid solution and Pr2MoO6. The monoclinic structure is restored after re-annealing at 1100 °C. For the sol–gel route, a fluorite structure emerges already at 600 °C and orders into monoclinic modification at 1100 °C. Rietveld refinement and IR spectroscopy indicate the possible formation of clusters of interconnected MoO6 octahedra. XANES spectroscopy and redox titration confirm the presents Pr4+ion with average content at room temperature of χ(Pr4+) ≈ 10 mol. %. Complex impedance measurements (200–900 °C, air) reveal bulk conduction with Ea = 0.64 eV, a grain-boundary contribution (Ea = 0.61 eV), and an electrode process that becomes diffusion-limited at high temperatures (Ea = 1.4 eV).