Phase relationships in cobalt iodate hydrates — thermal analyses, X-ray, IR, and Raman spectroscopic studies

The phase relationships of the cobalt iodate–water system have been examined using thermal analyses (DSC, TG), X-ray, IR and Raman spectroscopic methods. The compounds established are Co(IO3)2·4H2O, α-Co(IO3)2·2H2O, β-Co(IO3)2·2H2O, Co(IO3)2 as well as the novel compounds Co(IO3)2·xH2O and Co(IO3)2·H2O. β-Co(IO3)2·2H2O is isostructural to the respective nickel compound. Co(IO3)2·H2O crystallises orthorhombic with a=827.18(6) pm, b=1104.52(8) pm, and c=772.26(4) pm. The vibrational spectra display acentric space groups (violation of the mutual exclusion principle) for Co(IO3)2·H2O and Co(IO3)2. β-Co(IO3)2·2H2O dehydrates to Co(IO3)2, Co(IO3)2·xH2O at ∼300 K (TG) to Co(IO3)2·H2O, and Co(IO3)2·H2O at 519 K (DSC) to Co(IO3)2. Both the strength of the hydrogen bonds and the colour of the various cobalt iodate hydrates differ strongly. This is caused by the different ligand field splitting of the respective Co(IO3)x(H2O)y polyhedra. The former is shown from the average wave numbers of the uncoupled OD stretching modes (isotopically dilute samples), which decrease in the range Co(IO3)2·H2O (pale-pink, νOD: 2302 and 2248 cm−1, 90 K)<α-Co(IO3)2·2H2O (pink, νOD: 2453, 2423, and 2×2315 cm−1)