Paramagnetic Gd(IO3)3. Crystal structure of the transition metal iodates. IV

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Abstract
Gd(IO3)3, type I, crystallizes in the monoclinic system with space group P21/a and four formulas in the unit cell. The lattice constants, at 297 °K, are a=13.4365(9), b=8.5226(5), c=7.1356(5) Å and β=99.717(7) ° for λCuKα1=1.540598 Å. A total of 5012 independent reflections were measured on a CAD‐4 diffractometer, using Nb‐filtered MoKα radiation. The crystal structure was solved using a combination of Patterson and Fourier series, and was refined by the method of least squares. The final agreement factor R is 0.033. The iodate ions form a three‐dimensional network based on shared oxygen atoms, with the Gd3+ ion located within a distorted dodecahedron of oxygen atoms. The three independent IO−3 ions form triangular pyramids, with three short I–O bonds of average length 1.803 Å and average O–I–O angle of 98.7°. Each I atom has three additional oxygen neighbors at distances in the range 2.741–3.194 Å that complete its distorted octahedron. Gd(IO3)3, type I is the archetype for an isostructural family of of rare‐earth iodates extending from Ce to Lu. By contrast, no more than five (including Pm) rare‐earth ions are represented in any of the other five known types of anhydrous rare‐earth iodates. Small variations from monotonic in the lattice constants of type I anhydrous iodates are found, as predicted by the ’’tetrad’’ effect.