Paramagnetic Ni(IO3)2 · 2H2O. Crystal structure of the transition-metal iodates. I

Abstract

Nickel iodate dihydrate, paramagnetic at room temperature and weakly ferromagnetic below 3 °K, crystallizes in the orthorhombic system with space group Pbca and four formulas in the unit cell. The lattice constants at 298 °K are a = 9.14986 ± 0.00008, b = 12.20896 ± 0.00022, and c = 6.58353 ± 0.00013 Å. The crystal structure has been investigated independently in two laboratories. In one, the integrated intensities of 8883 reflections were measured with PEXRAD, within a hemisphere of reciprocal space of radius (sinθ)/λ = 1.02 Å−1. In the other, all independent reflections with (sinθ)/λ ≤ 0.70 Å−1 were measured on a Nonius automatic three-circle diffractometer. The crystal structure was solved by interpretation of Patterson and Fourier series, and refined by the method of least squares, using respectively 824 or 864 symmetry-independent structure factors. The final agreement factor is 0.047 for one set and 0.048 for the other, based on anisotropic temperature coefficients for all atoms except hydrogen, for which isotropic values were used. The structure consists of a network of corner-sharing IO6 octahedra with the Ni ions occupying octahedral interstices at the inversion centers. The water oxygen forms part of the octahedra around Ni, and is not directly bonded to I. The IO6 octahedra are highly distorted, with three I–O distances in the range 1.801–1.820 Å and three more in the range 2.804–2.935 Å. The NiO6 octahedra are almost regular, with Ni–O distances in the range 2.042–2.062 Å and O–Ni–O angles in the range 88.3–89.9°. The hydrogen atoms of the water molecule were detectable, and refinement gave acceptable molecular dimensions. Normal probability plot analysis was used to compare the results of the two independent structural investigations.