Lithium selenate is an inorganic compound of lithium and the selenate ion, with the chemical formula Li2SeO4. It is a colorless, water-soluble crystalline salt. It forms a well-characterized monohydrate, Li2SeO4·H2O, which is a polar and pyroelectric crystal.
Preparation Lithium selenate can be prepared by neutralizing selenic acid with lithium carbonate:
Li2CO3 + H2SeO4 → Li2SeO4 + CO2↑ + H2O
Crystallization from aqueous solution generally gives the hydrated salt. Large single crystals of the monohydrate have been grown from aqueous solution at about 343 K.
Properties Structure Anhydrous Li2SeO4 crystallizes in the trigonal crystal system, in space group R (No. 148). Its crystal structure was refined by Pertlik and Fuith in 1989.
The structure consists of isolated SeO42− tetrahedra connected through lithium–oxygen coordination polyhedra.
Monohydrate Lithium selenate monohydrate, Li2SeO4·H2O, crystallizes in the monoclinic space group P21 (No. 4). At room temperature its lattice parameters are approximately a = 5.580 Å, b = 5.028 Å, c = 8.452 Å and β = 107.63°, with two formula units per unit cell.
It is isostructural with lithium sulfate monohydrate. The crystal structure contains LiO4 and LiO3(H2O) tetrahedra together with SeO4 tetrahedra. These share oxygen vertices to form a three-dimensional framework.
Because space group P21 is polar, the monohydrate is pyroelectric. Its pyroelectric coefficient and thermal expansion have been measured between approximately 183 and 343 K.
The crystal is also optically anisotropic. Refractive indices have been measured over wavelengths from 365 to 1530 nm, and calculations indicate that Li2SeO4·H2O permits both type-I and type-II phase matching for second-harmonic generation from wavelengths of roughly 650 nm into the near-infrared.
Thermal behaviour Hydrated lithium selenate loses water on heating. Thermogravimetric studies of hydrated samples found dehydration mainly between about 120 and 250 °C.
At still higher temperature, molten Li2SeO4 is progressively reduced to lithium selenite, Li2SeO3, between approximately 650 and 850 °C.
Li2SeO4 → Li2SeO3 + O2
Above about 900 °C, further mass loss occurs through decomposition and volatilization of the selenite; lithium oxide remains among the final solid products.
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