Structure cristalline et expansion thermique de l'iodure de nickel hexahydrate

The structure of NiI ₂, 6 H ₂O has been determined by X-ray diffraction techniques. The dimensions of the hexagonal unit cell are: a = 7.638 ± 0.005 Å and c = 4.876 ± 0.005 Å, with Z = 1. The space group is Poverline3m1 . The structure was deduced from Patterson and Fourier syntheses and refined by least-squares method to a final R value of 0.09. It is almost a layer structure, each layer is composed by n complex ions [Ni(H ₂O) ₆] ²⁺ surrounded by 2 n ions I ^-; this is in agreement with the good cleavage of crystals and with the marked anisotropy in the thermal expansion. But the structure may also be regarded as a derivation of NiAs structure, with only {1}/{8} of octahedral holes occupied. A structural classification of compounds MX ₂, 6 H ₂O is proposed.