Holmium arsenate is an inorganic compound of holmium and the arsenate ion, with the chemical formula HoAsO4. It is a very pale pinkish-white, sparingly soluble crystalline solid and adopts the tetragonal xenotime structure type.
Preparation Holmium arsenate can be prepared by precipitation from aqueous solutions containing holmium(III) and arsenate ions. For example, holmium(III) chloride reacts with sodium arsenate:
HoCl3 + Na3AsO4 → HoAsO4↓ + 3 NaCl
This preparation has been reported as part of systematic syntheses of rare-earth arsenates.
A reference sample for powder-diffraction measurements was prepared from an aqueous solution of arsenic pentoxide and holmium trichloride.
Properties Crystal structure Holmium arsenate crystallizes in the tetragonal crystal system, space group I41/amd (No. 141), with the xenotime or zircon structure type.
At 25 °C, the lattice parameters are a = 7.0548 Å, c = 6.3159 Å with four formula units per unit cell. The calculated density is 6.420 g/cm3.
The structure consists of isolated tetrahedra and eight-coordinate Ho3+ ions. The HoO8 polyhedra form edge-sharing chains that are linked by arsenate tetrahedra into a three-dimensional framework.
Reported interatomic distances are approximately 1.684 Å for As–O and 2.294–2.422 Å for Ho–O.
Solubility Holmium arsenate is very sparingly soluble in water. A concentration-based solubility-product value of pKsp,c = has been reported.
Vibrational properties Polarized Raman spectroscopy of HoAsO4 shows the characteristic internal vibrations of the AsO4 tetrahedra and lattice modes of the xenotime structure.
References