Holmium phosphate is an inorganic compound of holmium and phosphate with the chemical formula HoPO4. Hydrated forms are also known.
Preparation Hydrated holmium phosphate can be prepared by precipitation from aqueous solutions containing Ho3+ and phosphate ions. A dihydrate has been obtained by reaction of holmium(III) nitrate with dilute phosphoric acid.
Anhydrous HoPO4 single crystals can be grown from a PbO–P2O5 flux using holmium(III) oxide and ammonium dihydrogen phosphate.
Structure and properties Anhydrous holmium phosphate crystallizes in the tetragonal crystal system with the xenotime structure, which is isostructural with zircon. It belongs to space group I41/amd (No. 141).
Its structure consists of isolated PO4 tetrahedra and eight-coordinate Ho3+ ions. Alternating HoO8 and PO4 polyhedra form chains parallel to the crystallographic c axis.
Hydrated holmium phosphate has also been reported in an orthorhombic form. Early powder-diffraction work assigned this hydrated phase to space group P222, with eight formula units per unit cell. Other work on hydrated rare-earth orthophosphates found HoPO4 hydrates to be closely related to tetragonal xenotime-type structures, with water occupying interstitial positions.
Magnetic properties HoPO4 is strongly magnetically anisotropic at low temperature. Magnetic susceptibility and heat-capacity measurements show an antiferromagnetic transition at approximately 1.39 K.
The magnetic behaviour is approximately Ising-like, with the easy axis along the tetragonal c direction. The ordered state has been described as a simple two-sublattice antiferromagnetic structure.
Crystal-field spectroscopy and magnetic-susceptibility measurements have also been used to determine the low-lying Ho3+ crystal-field levels in HoPO4.
High-pressure behaviour Single-crystal X-ray diffraction and first-principles calculations show that xenotime-type HoPO4 remains mechanically stable over a substantial pressure range. Under compression, its elastic anisotropy increases and the material is more resistant to volume compression than to shear deformation.
Reactions At high temperature, HoPO4 reacts with sodium molybdate to form the mixed phosphate molybdate Na2Ho(MoO4)(PO4).
The resulting Na2Ho(MoO4)(PO4) is orthorhombic and belongs to a family of layered rare-earth phosphate molybdates.
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