Synthesis, structure and properties of triple molybdates of the K₅RZr(MoO₄)₆ composition in K₂MoO₄ – R₂(MoO₄)₃ – Zr(MoO₄)₂ systems (R = trivalent elements)

A comprehensive physico-chemical study of individual systems provides important experimental data, which can be further used both in basic and applied research. Thus, a number of systematic studies have revealed the formation of complex molybdates, which can be classified in terms of cationic composition. Many compounds belonging to this class are shown to possess valuable functional properties. In this work, we set out to generalise and complement the results obtained on phase formation in K2MoO4 – R₂(MoO₄)₃ – Zr(MoO₄)₂ salt systems (R = Al, Cr, Fe, In, Sc, Y, Bi, La –Lu) dependent on the nature of the molybdates of trivalent elements. In addition, we aim to determine synthesis conditions and to characterise properties of these triple molybdates. To this end, methods of X-ray phase analysis, differential scanning calorimetry, electron microscopy and impedance spectroscopy were used. Interaction in ternary salt systems K₂MoO₄ – R₂(MoO₄)₃ – Zr(MoO₄)₂ (R = Al, Cr, Fe, In, Sc, Y, Bi, La – Lu) was studied at subsolidus temperatures across the 723–873 K range. Transformation of phase diagrams was demonstrated; the systems were divided into ten triangulation types. The identified new triple molybdates of the K₅RZr(MoO₄)₆ (R = Al, Cr, Fe, In, Sc, Y, Bi, Dy - Lu) composition were obtained via solid-phase synthesis at temperatures of 723–773 K. Their crystallographic and thermal properties were determined. Molybdates of the K₅RZr(MoO₄)₆ composition are established to crystallize in a hexagonal crystal system and a trigonal crystal system under R = Al, Cr, Fe, In, Sc (space group P63, Z = 2) and under R = Dy - Lu, Y, Bi (space group R`3c, Z = 6), respectively. The electrical conductivity of K₅RZr(MoO₄)₆ (R = Fe, In, Er) was studied using the method of impedance spectroscopy across the temperature range of 300–900 K. It is also demonstrated that the conductivity values of this compound amount to 10^-2 S/cm at 700–850 K.

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