Dysprosium arsenate is an inorganic compound of dysprosium and the arsenate ion, with the formula DyAsO4. It is a sparingly soluble crystalline solid with the tetragonal xenotime structure type at room temperature. At low temperature it undergoes a cooperative Jahn–Teller structural transition followed by antiferromagnetic ordering.
Preparation Dysprosium arsenate can be prepared by precipitation from aqueous solutions of a dysprosium(III) salt and a soluble arsenate. For example, reaction of dysprosium(III) chloride with sodium arsenate gives DyAsO4:
DyCl3 + Na3AsO4 → DyAsO4↓ + 3 NaCl
This preparation has been reported for rare-earth arsenates in aqueous solution.
A sample used for standard powder-diffraction measurements was prepared from an aqueous solution of dysprosium trichloride and arsenic pentoxide and dried at 110 °C.
Properties Crystal structure At room temperature, DyAsO4 crystallizes in the tetragonal crystal system, space group I41/amd (No. 141), with the xenotime or zircon structure type.
At 25 °C, lattice parameters of
a = 7.0733 Å c = 6.3133 Å
have been reported, with four formula units per unit cell. The corresponding calculated density is 6.338 g/cm3.
The structure contains isolated tetrahedra and eight-coordinate Dy3+ ions. As in other heavy rare-earth arsenates, the DyO8 polyhedra and AsO4 tetrahedra form a three-dimensional framework.
Solubility Dysprosium arsenate is very sparingly soluble in water. A concentration-based solubility-product value of
pKsp,c = 23.83 ± 0.06
has been reported.
Low-temperature phase transition At approximately 11.2 K, DyAsO4 undergoes a cooperative Jahn–Teller structural transition from the tetragonal phase to an orthorhombic phase.
Low-temperature X-ray diffraction shows that the distortion occurs primarily in the basal plane: the tetragonal a parameter splits into unequal orthorhombic a and b axes, whereas the c parameter remains nearly unchanged. The low-temperature structure has been described in space group Imma, with representative cell dimensions near a = 7.06 Å, b = 7.03 Å and c = 6.30 Å.
The transition is driven by quadrupolar interactions associated with the dysprosium ions and is analogous to the cooperative Jahn–Teller transition in DyVO4.
Magnetic properties DyAsO4 undergoes a second transition at about 2.44 K, where long-range antiferromagnetic order develops.
Specific-heat measurements show λ-type anomalies at both 11.2 K, corresponding to the structural transition, and at the Néel temperature of about 2.44 K.
Neutron diffraction shows that the low-temperature magnetic structure is non-collinear and antiferromagnetic, with the Dy moments lying predominantly in the basal plane. At 2.2 K the ordered moment is about 8.6 μB per Dy ion.
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