Cobalt(II) selenate is an inorganic compound of cobalt and the selenate ion, with the chemical formula CoSeO4. The anhydrous compound is a violet crystalline solid. It is soluble in water and forms several crystalline hydrates.
Preparation Cobalt(II) selenate can be prepared by neutralizing selenic acid with cobalt(II) carbonate or basic cobalt carbonate:
CoCO3 + H2SeO4 → CoSeO4 + CO2↑ + H2O
A modern preparation used basic cobalt carbonate, Co2CO3(OH)2, and selenic acid at about 70 °C. Slow evaporation at room temperature produced dark red CoSeO4·6H2O crystals. Heating the hexahydrate at 125 °C yielded the monohydrate, and prolonged heating at 315 °C gave anhydrous CoSeO4.
Properties Structure Anhydrous CoSeO4 crystallizes in the orthorhombic crystal system, in space group Pnma (No. 62). At 300 K, neutron diffraction gives lattice parameters a = 9.0421 Å, b = 6.7669 Å and c = 4.8860 Å.
The structure is analogous to β-CoSO4. It consists of chains of edge-sharing CoO6 octahedra, with neighboring chains linked through SeO42− tetrahedra. The octahedral chains are alternately tilted relative to the crystallographic axes.
Older literature sometimes describes the same structure in the non-standard space-group setting Pbnm.
Hydrates Several hydrates of cobalt(II) selenate are known, including the mono-, tetra-, penta-, hexa- and heptahydrates. Thermal-dehydration studies have also identified a dihydrate as an intermediate phase.
The pentahydrate, CoSeO4·5H2O, crystallizes in the triclinic system, space group P, with lattice parameters a = 6.435 Å, b = 10.703 Å, c = 6.220 Å, α = 98.71°, β = 109.60° and γ = 75.54°, with two formula units per unit cell.
It is isostructural with CuSO4·5H2O. Four water molecules coordinate directly to cobalt, while the fifth occupies an interstitial position in the hydrogen-bonded structure.
The tetrahydrate, CoSeO4·4H2O, is monoclinic, space group P21/n, with lattice parameters approximately a = 6.00 Å, b = 13.84 Å, c = 8.06 Å and β = 90.9°.
Thermal behaviour Dehydration of CoSeO4·6H2O proceeds stepwise. The pentahydrate, tetrahydrate, dihydrate and monohydrate have all been detected as intermediate phases before formation of anhydrous CoSeO4.
At still higher temperatures, anhydrous cobalt selenate decomposes through lower selenium oxyanion phases, ultimately yielding cobalt oxide while selenium oxides and oxygen are evolved.
Magnetic and dielectric properties CoSeO4 is an antiferromagnetic chain compound. Magnetic susceptibility and neutron-diffraction measurements show the development of long-range magnetic order below about 30 K.
In the ordered state, spins along each cobalt chain are predominantly antiferromagnetically aligned, with neighboring chains also coupled antiferromagnetically. At 5 K the ordered cobalt moment is about 3.5 μB.
Application of a magnetic field below the ordering temperature produces a field-induced spin reorientation toward the crystallographic c axis. The dielectric constant changes slope at the magnetic ordering temperature, and its field dependence changes strongly across the spin-reorientation transition, indicating coupling between magnetic and dielectric degrees of freedom.
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