Neodymium(III) selenate is an inorganic compound of neodymium and the selenate ion, with the chemical formula Nd2(SeO4)3. Several hydrated forms are known, including the pentahydrate, octahydrate and dodecahydrate.
Preparation Neodymium(III) selenate can be prepared by dissolving neodymium(III) oxide in aqueous selenic acid:
Nd2O3 + 3 H2SeO4 → Nd2(SeO4)3 + 3 H2O
Concentration of the resulting solution at elevated temperature gives pink crystals of the pentahydrate, Nd2(SeO4)3·5H2O.
Single crystals of the pentahydrate have also been obtained by dissolving Nd2O3 in concentrated aqueous selenic acid and crystallizing the resulting solution.
Properties The pentahydrate forms pink crystals and is the thermodynamically stable neodymium selenate hydrate in contact with water at 25 °C.
Older work also identified an octahydrate and a dodecahydrate. The octahydrate can be obtained by adding a saturated neodymium selenate solution to excess ethanol, while the dodecahydrate forms initially on slow evaporation of saturated solution at room temperature. The dodecahydrate readily loses water to form the octahydrate.
The measured solubility of neodymium selenate decreases with increasing temperature. Reported values include approximately 45.2 g per 100 g water at 0 °C and about 39.9 g per 100 g water at 40 °C, although published solubility measurements show significant disagreement depending on hydrate composition and experimental method.
The OECD thermodynamic review considers Nd2(SeO4)3·5H2O to be the stable phase at 298.15 K, but does not recommend a single solubility value because published measurements range from about 0.55 to 0.92 mol kg−1.
Crystal structure Neodymium(III) selenate pentahydrate crystallizes in the monoclinic crystal system, in space group P21/c (No. 14). Its lattice parameters at room temperature are approximately a = 10.629 Å, b = 13.98 Å and c = 9.752 Å, with β = 93.13° and four formula units per unit cell.
The structure contains Nd3+ ions linked to SeO4 tetrahedra and coordinated water molecules. Although its structural motifs resemble those of rare-earth sulfate pentahydrates, Nd2(SeO4)3·5H2O is not isostructural with the corresponding neodymium sulfate hydrate.
Related compounds In concentrated selenic acid, neodymium can also form neodymium(III) hydrogen selenate, Nd(HSeO4)3.
Double selenates containing neodymium are also known, including KNd(SeO4)2, which contains layers built from NdO9 polyhedra decorated by SeO4 tetrahedra.
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