Fluorite-like phases in the La2MoO6 – Sm2MoO6 – MoO3 system: Homogeneity region, crystal structure, and conductive properties

Abstract

Samples in the La₂MoO₆ – Sm₂MoO₆ – MoO₃ system were obtained using the solid-state synthesis method. The phase formation in the given compositional section was firstly studied, and the homogeneity region of the fluorite-like lanthanum-samarium molybdate was determined, as confirmed by XRD analysis including Rietveld refinement and SEM with energy-dispersive microanalysis. The fluorite-like phase contains less molybdenum than the isostructural neodymium molybdate Nd₅Mo₃O_16+δ. The homogeneity region of fluorite-like solid solutions has been determined, which corresponds to the La_5-xSm_xMo_2.75O_15.75 section in the composition range 2.5 ≤ x ≤ 3.5. The crystal structure is characterized by the splitting of cation positions and the presence of excess oxygen in the octahedral voids of the structure. Oxygen atoms in the octahedral positions are displaced from the center of the voids and enter into the molybdenum coordination sphere. The density of single-phase ceramic samples increases with the increase in samarium content. The conductive properties of single-phase ceramics were investigated in an air atmosphere using impedance spectroscopy. The total conductivity was ∼10⁻² S cm⁻¹ for La_2.5Sm_2.5Mo_2.75O_15.75 at 800 °C. The grain bulk conductivity exhibits a break at a temperature of approximately 600 °C, which may be explained by the depletion of charge carriers in the structure.