Synthesis, crystal structure, and thermal studies of (NH 4) 2Cd 2(SeO 4) 3·3H 2O

The species (NH ₄) ₂Cd ₂(SeO ₄) ₃^.3H ₂O has been synthesized and its crystal structure has been determined. Crystals were obtained by slow evaporation from an aqueous solution at 85-90°C. The compound is monoclinic, {P2 ₁}/{m}, a = 6.836(2), b = 19.372(4), c = 5.690(2) Å, β = 94.02(2)°, V = 751.7(6) Å ³, D_x = 3.285 g cm ^-3, Z = 2; F(000) = 692.0, λ(Mo Kα) = 0.71069Å, μ(Mo Kα) = 106.99 cm ^-1, 298 K. Refinement was terminated at R = 0.043 for all observed reflections. Two kinds of tetrahedrally coordinated selenium atoms occur in the structure which form double chains which in turn give the three-dimensional structure. The cadmium atoms are coordinated by five oxygen atoms from different selenate anions, and by an oxygen atom from a water molecule. This structure gives cavities, each one accommodating a water molecule and two ammonium groups. Thermogravimetry shows that the anhydrous species of langbeinite stoichiometry is not stable.