Synthesis and characterization of the novel rare earth orthophosphates Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄

The new mixed rare earth (RE) orthophosphates Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄ were synthesized by a classical solid state reaction in an electrical furnace at 1200 C. As starting materials, the corresponding rare earth oxides and diammonium hydrogen phosphate were used. The powder diffraction analyses revealed that the new compounds Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄ crystallize in a zircon-type structure being isostructural with the rare earth orthophosphate YPO₄. Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄ crystallize in the tetragonal space group I4₁/amd (no. 141) with four formula units in the unit cell. The structural parameters based on Rietveld refinements are a = 687.27(2), c = 601.50(2) pm, V = 0.28412(1) nm³, R_p = 0.0143, and R_wp = 0.0186 (all data) for Y_0.5Er_0.5PO₄ and a = 684.61(2), c = 599.31(2) pm, V = 0.28089(2) nm³, R_p = 0.0242, and R_wp = 0.0313 (all data) for Y_0.5Yb_0.5PO₄. Furthermore, the structure of Y_0.5Er_0.5PO₄ was refined from single-crystal X-ray diffraction data: a = 687.78(5), c = 601.85(4) pm, V = 0.28470(5) nm³, R₁ = 0.0165, and wR₂ = 0.0385 (all data). In both compounds, the rare earth metal ions are eightfold coordinated by oxygen atoms, forming two unique interlocking tetrahedra with two individual RE-O distances. The tetrahedral phosphate groups [PO₄]^3- are slightly distorted in both compounds. The individual rare earth ions share a common position (Wyckoff site 4a). The presence of two rare earth ions in the structures of the new orthophosphates Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄ was additionally confirmed by single-crystal EDX spectroscopy revealing a ratio of 1:1.