Structural features of sodium- and halogen-containing fluorite-like rare-earth molybdates

Compounds of nominal composition NaLa₄Mo₃O₁₅F_1-Cl (x = 0-0.7) have been obtained for the first time as ceramics and single crystals. All the compounds belong to the cubic crystal system and are isostructural to undoped crystals of the Ln₅Mo₃O₁₆ family (Ln = La-Gd). A study of the physical properties of the obtained phases has shown their significant change compared to undoped compounds and a reversible jump in conductivity near 600 °C. According to X-ray diffraction analysis, sodium atoms are localized near the positions of rare-earth cations La2, and halogen impurities enter partially occupied general positions of O2 atoms and the interstices formed by six La1 atoms. As the temperature increases, chlorine ions leave O2 positions and interstices. This increases the number of vacancies and creates new ion transport pathways. A jump in conductivity occurs when three conditions are simultaneously met: an increase in the number of vacancies, the appearance of new charge transport pathways, and, possibly, an increase in the mobility of the main charge carriers, oxygen ions, due to electrostatic repulsion between anions (O^2-, F^-, Cl^-).