ИСТИНА

The Ln2W(Mo)O6 compounds, the so-called oxymolybdates and oxytungstates, have a different structural type depending on the ionic radius of the rare-earth cation and the synthesis temperature. The oxymolybdates Ln2MoO6 with large rare-earth elements Ln = La, Pr, Nd, synthesized at temperatures above 1000°, have a layered tetragonal structure with space group I41/acd [1] or I-42m [2] at room temperature. Doping of such compounds with bivalent lead led to a significant improvement in the conductive properties and the appearance of polymorphism [3]. In the present work, the effect on the phase formation, structure, and physical properties of the Nd2MoO6 compound upon its doping with bivalent magnesium was studied. Polycrystalline samples of (MgO)x(Nd2MoO6)(1-x)/2 were prepared by solid-phase synthesis in air at a temperature of 1250°C, where x = 0, 0.3, 0.05, 0.10, 0.125, 0.15, 0.20. Single crystals of Nd2MoO6, pure and doped with magnesium, were grown by the flux method. The resulting mono- and polycrystalline samples were characterized by X-ray phase analysis, EDS microanalysis, X-ray diffraction analysis, differential scanning calorimetry, thermogravimetry, and impedance spectroscopy. The study of powder X-ray diffraction patterns of polycrystalline samples (MgO)x(Nd2MoO6)(1-x)/2 (x = 0, 0.03, 0.05, 0.10, 0.125, 0.15, 0.20) showed that the region of solid solutions with the tetragonal Nd2MoO6 structure extends to x = 0.20. EDS microanalysis confirmed the entry of magnesium into the Nd2MoO6 crystal lattice. The study of samples using the DSC method did not reveal any anomalies in the entire temperature range under study. X-ray analysis of the pure and doped monocrystalline samples showed that the model of the structure in which magnesium atoms partially replace molybdenum atoms best fit the observed diffraction pattern. The study of the electrophysical properties of the synthesized polycrystalline Nd2MoO6 samples with different magnesium content by impedance spectroscopy revealed a significant difference in the magnitude and nature of the conductivity of the samples during cooling and heating. The difference gradually disappears with each new heating-cooling cycle, which may indicate instability of the oxygen sublattice at high temperatures. This work was supported by Russian Foundation for Basic Research (project No. 18-29-12005). [1] Efremov V. A., Tyulin A. V., Trunov V.K. Koordinatsionnaya Khimiya, 1987, 13, 1276-1282. [2] Sillen L.G., Lundborg K.La2MoO6, ein Lanthanoxymolybdat mit Schichtenstruktur. Z. Anorg. Chem., 1943, 252, 2-8. [3] Voronkova V.I., Orlova E.I., Kazakov S., Kharitonova E.P., Belov D.. Phase relations and physical properties of layered Pb-containing Nd2MoO6 compounds. Eur. J. Inorg. Chem., 2016, 1022-1029.