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

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.