Thulium arsenate is an inorganic compound of thulium and the arsenate ion, with the formula TmAsO4. It is a sparingly soluble crystalline solid with the tetragonal xenotime structure type. At very low temperature it undergoes a cooperative Jahn–Teller structural transition.
Preparation Thulium arsenate can be prepared by precipitation from aqueous solutions containing Tm3+ and arsenate ions.
Single crystals have also been grown by a high-temperature flux method. In one procedure, thulium(III) oxide was reacted in molten lead pyroarsenate, Pb2As2O7, at approximately 1300 °C.
Properties Crystal structure At room temperature, TmAsO4 crystallizes in the tetragonal crystal system with the xenotime/zircon structure type, space group I41/amd (No. 141), with four formula units per unit cell.
Room-temperature lattice parameters of approximately a = 6.9939 Å and c = 6.2595 Å have been reported.
The structure consists of isolated tetrahedra and eight-coordinate Tm3+ ions. TmAsO4 belongs to the heavy rare-earth arsenate series, for which the xenotime structure is favored over the monoclinic monazite structure.
Polarized Raman spectroscopy shows the internal vibrational frequencies of the AsO4 tetrahedron increasing across the rare-earth arsenate series as the rare-earth ionic radius decreases.
Solubility Thulium arsenate is sparingly soluble in water. Firsching reported a concentration-based solubility-product value of
pKsp,c = 23.08 ± 0.01
for TmAsO4.
Low-temperature phase transition TmAsO4 is a well-studied cooperative Jahn–Teller material. At room temperature it is tetragonal and paramagnetic, but below a critical temperature of approximately 6.13 K it undergoes a symmetry-lowering distortion to an orthorhombic phase.
The transition arises because the crystal field leaves the lowest Tm3+ electronic state as a doublet. The cooperative Jahn–Teller distortion removes this degeneracy and creates ferroelastic domains with principal axes along the and directions.
Optical absorption, magnetic susceptibility and enhanced 169Tm NMR measurements show that the size of the Jahn–Teller splitting below the transition closely follows mean-field behavior.
Applied stress or magnetic field can alter the balance between the distorted domain states. First-order transitions between different distortion modes have been observed below the Jahn–Teller transition temperature.
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