GADOLINIUM(III) COMPLEX OXIDE COMPOUNDS OF OBTAINED FROM MOLYBDATE-PHOSPHATE MOLTEN SALT

The peculiarities of K2Gd(PO4)(MoO4) single crystal growth from combined phosphate-molybdate melts have been investigated. The formation regions of GdPO4,  KGd(MoO4)2 and their cocrystallization fields with the mixed anionic compound potassium – gadolinium have been established. IR spect­roscopy showed that in all compounds molybdenum and phosphorus are in a tetrahedral environment, and the phase composition was confirmed by X-ray powder. Crystallization of the target compound K2Gd(PO4)(MoO4) is observed in the pseudobinary section K2Mo2O7-KPO3 orthiorombic system, space group Ibca, Z = 8, а = 19,694(4) Å, b = 12,260(3) Å, c = 6,961(3) Å. Double phosphate K3Gd(PO4)2 crystallizes in the form of colorless prismatic crystals from the melts with the initial ratio K/P = 1.7–3.3, which practically corresponds to the section K2MoO4 – K4P2O7. Phosphate GdPO4 crystallizes in the form of colorless prismatic crystals up to 0.1 mm in size at the maximum KPO3 content in the melt, which corresponds to K/P ratio = 0.20–0.50 and K/(Mo+P) = 0.25–0.33. Crystal-chemical criteria for the formation of mixed-anion-type layered frameworks, island structures, and three-dimensional frameworks based on mixed phosphate-molybdate systems and rare earth elements are proposed. The key factor in the formation of complex oxide compounds of rare earth elements from combined molybdate-phosphate melts is the ratio of K/Mo in the initial solution - melt: 1) at  K/Mo = 0.5–1.1, LnPO4 orthophosphates are formed, which are characterized by a 3D framework on the basis of LnO7/LnO8poly­hedra condensed into a three-dimensional framework; at K/Mo = 1.2–1.5, mixed anionic compounds of the composition K2Ln(PO4)(MoO4) (Ln – Gd- Tb) with a layered structure are formed, where LnО8 forms zigzag chains; 3) at K/Mo = 1.6–3.0, double orthophosphates of the composition K3Ln(PO4)2 with an island structure are formed.