Ytterbium(III) arsenate is an inorganic compound of ytterbium in the +3 oxidation state and the arsenate ion, with the formula YbAsO4. It is a colorless, sparingly soluble crystalline solid with the tetragonal xenotime structure type.
Preparation Ytterbium(III) arsenate can be precipitated from aqueous solutions containing ytterbium(III) and arsenate ions. For example, ytterbium(III) chloride reacts with sodium arsenate:
YbCl3 + Na3AsO4 → YbAsO4↓ + 3 NaCl
This method has been reported as part of systematic preparations of rare-earth arsenates.
An NBS reference sample was prepared by precipitation from an aqueous solution of arsenic pentoxide and ytterbium trichloride, followed by heating for one hour at 880 °C.
Single crystals have also been obtained unintentionally during high-temperature reactions involving Yb2O3, YbCl3 and As2O3. In one experiment, air entered the reaction system and oxidized arsenic(III) to arsenic(V), producing YbAsO4 as the main crystalline product.
Properties Crystal structure YbAsO4 crystallizes in the tetragonal crystal system, space group I41/amd (No. 141), with the xenotime or zircon structure type.
At 293 K, single-crystal X-ray diffraction gives a = 6.9712(4) Å, c = 6.2431(4) Å with four formula units per unit cell and a calculated density of 6.830 g/cm3.
The structure consists of chains of edge-sharing YbO8 coordination polyhedra linked by isolated AsO4 tetrahedra. Ytterbium is eight-coordinate, with two distinct Yb–O distances of approximately 2.258 and 2.391 Å, while the As–O distance is about 1.683 Å.
Earlier powder-diffraction work had already identified YbAsO4 as a zircon-type phase with the same space group.
Solubility Ytterbium(III) arsenate is very sparingly soluble in water. A concentration-based solubility-product value of pKsp,c = 22.72 has been reported.
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