COEXISTENCE OF CERIUM(III)/(IV) IN PHOSPHATE AND MOLYBDATE MELTS

Анотація

For the first time, the possibilities of controlled in fluence on the valence states of ceriumin phosphate and molybdate melts have been systematically analyzed. A series of molybdates with a scheelite structure, M^ICe(MoO₄)₂ (M^I – Li, Na, K, Cs), were obtained by crystallization from melt solutions, and the crystallization regions of Na₃Ce(PO₄)₂ were established. The role of the acidity of molybdate, phosphate-molybdate, and fluoride-phosphate melts in the stability of cerium compounds in different oxidation states was determined for the first time. The role of acidity in molybdate, phosphate-molybdate, and fluoride-phosphate melts on the stability of cerium compounds in various oxidation states has been established. In more acidic melts corresponding to the salts M^I₂Mo₃O₁₀ and M^I₂Mo₂O₇ (M^I = Li, Na, K, Cs) with low M/Moratios (0.5–0.8), stabilization of the lower oxidation state of cerium (III) is observed. To optimize the conditions for obtaining monocrystalline doublephosphate Na₃Ce(PO₄)₂ under milder conditions and at lower temperatures, we selected two systems: Na₂O–P₂O₅–NaF and Na₂O–P₂O₅–MoO₃. The salts NaF and Na₂MoO₄ were chosen because their presence in phosphate melts is one of the factors that depolymerize phosphate tetrahedra and lower the melting temperature of the corresponding batch due to the formation of low-melting eutectics. Thus, with a sodium fluoride content ranging from 30 to 60 mol.%, the exclusive formation of Na₃Ce(PO₄)₂ is observed. Thus, during spontaneous crystallization from fluoridemelts, the Na/P ratio plays a decisive role, while the influence of the fluoride component becomes structurally formative at NaF concentrations between 30 and 60%. Conversely, with an increase in the content of alkali elements in the molten salts, the formation of a phase with a high eroxidation state of cerium in the form of CeO₂ occurs. The established pattern can be used to obtain a broader range of inorganic compounds based on cerium (III).