Hafnium(IV) selenate is an inorganic compound of hafnium and the selenate ion, with the chemical formula Hf(SeO4)2. Both anhydrous and hydrated forms are known; the tetrahydrate Hf(SeO4)2·4H2O has been characterized by single-crystal X-ray diffraction.
Preparation Hafnium(IV) selenate tetrahydrate can be prepared hydrothermally from hafnium(IV) oxide and concentrated selenic acid. A synthesis at 300 °C for six hours using HfO2 and 40% H2SeO4 yields Hf(SeO4)2·4H2O.
HfO2 + 2 H2SeO4 + 2 H2O → Hf(SeO4)2·4H2O
The tetrahydrate has also been prepared by reaction of selenic acid with saturated solutions of hafnium oxychloride hydrate.Davidovich, R. L.; Medkov, M. A. "Zirconium and hafnium diselenates". Koordinatsionnaya Khimiya. 1975. 1 (11): 1478–1480.
Structure and properties Hafnium(IV) selenate tetrahydrate crystallizes in the orthorhombic crystal system, in space group Fddd (No. 70). At 290 K its lattice parameters are a = 5.6431(4) Å, b = 11.9463(6) Å and c = 26.5207(14) Å, with eight formula units per unit cell.
Each Hf4+ ion is coordinated by eight oxygen atoms: four from selenate ions and four from coordinated water molecules. The compound is isostructural with Hf(SO4)2·4H2O and closely related to Zr(SeO4)2·4H2O.
Infrared spectroscopy shows the characteristic vibrational modes of coordinated selenate groups and water. Hirshfeld-surface analysis of the tetrahydrate indicates that O-O contacts make the largest contribution to intermolecular interactions in the crystal.
Thermal decomposition On heating, the tetrahydrate loses water to form anhydrous Hf(SeO4)2. Older thermal studies report dehydration in the approximate range 140–220 °C.
The anhydrous selenate is stable to substantially higher temperature but decomposes at about 610 °C.
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