Ytterbium(III) phosphate is an inorganic compound of ytterbium and phosphate with the chemical formula YbPO4. Hydrated forms are also known.
Preparation Hydrated ytterbium(III) phosphate can be prepared by precipitation from aqueous solutions containing Yb3+ and phosphate ions. A dihydrate has been obtained by reaction of ytterbium(III) chloride with phosphoric acid.
Anhydrous YbPO4 can also be prepared from ytterbium(III) oxide using silicon pyrophosphate, SiP2O7:
SiP2O7 + Yb2O3 → 2 YbPO4 + SiO2
Single crystals have also been grown from Yb2O3 and ammonium dihydrogen phosphate at high temperature.
Structure and properties Anhydrous ytterbium(III) phosphate crystallizes in the tetragonal crystal system with the xenotime structure, in space group I41/amd (No. 141). The structure consists of isolated PO4 tetrahedra and eight-coordinate Yb3+ ions.
Representative room-temperature lattice parameters are a = 6.816 Å and c = 5.966 Å, with four formula units per unit cell. The calculated density is approximately 6.44 g/cm3.
Hydrated ytterbium(III) phosphate can be converted to the anhydrous xenotime phase by calcination. Nanosized hydrated powders transform into well-crystallized xenotime-type YbPO4 above about 800 °C.
Optical properties Ytterbium(III) phosphate shows near-infrared absorption and emission associated with the 4f transitions of Yb3+. In well-crystallized xenotime-type material, the 2F7/2 and 2F5/2 manifolds are split into distinct Stark levels, producing sharp optical features.
The near-infrared photoluminescence intensity increases significantly when hydrated material is calcined to form the ordered xenotime structure.
Cooperative optical transitions involving Yb3+–Yb3+ pairs have also been observed in YbPO4 crystals.
Applications Ytterbium(III) phosphate has been investigated as a host material for near-infrared phosphors and for incorporation into glass–ceramic optical materials. Its chemical stability allows YbPO4 particles to survive incorporation into phosphate and silica glass matrices with little reaction.
More recently, xenotime-type YbPO4 has been investigated as an environmental barrier coating material for SiC-based ceramic composites because of its thermal expansion behaviour and resistance to steam and molten silicate attack.
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