Crystal structure and optical absorption spectra of LiCo(SO4)OH and its remarkable relationship to the Zn-Mn-silicate hodgkinsonite

Crystals of the compound LiCo(SO 4 )OH were synthesised at low-hydrothermal conditions, and the crystal structure was determined and refined from single crystal X-ray diffraction data. LiCo(SO 4 )OH crystallises monoclinic, space group P 2 1 / c , Z = 4, a = 9.586(2), b = 5.425(1), c = 7.317(1) Å, β = 109.65(1)°, V = 358.3 Å 3 , wR 2 = 0.0485 (2215 unique reflections, 78 variables). The structure is built from chains of edge-sharing, quite strongly bond-length and -angle distorted Co(OH) 3 O 3 octahedra ( = 2.126 Å), which are further linked by common corners, hydrogen bonds, and by properly shaped SO 4 tetrahedra ( = 1.476 Å) to sheets parallel (100). These sheets are connected to a three-dimensional framework by sharing corners with distorted LiO 4 polyhedra ( = 1.956 Å). Apart from the isotypic sulfates of Mn 2+ and Fe 2+ , only the molybdate LiCd(MoO 4 )OH crystallises isostructural with LiCo(SO 4 )OH. However, a very close structural relationship exists with the rare mineral hodgkinsonite, Zn 2 Mn(SiO 4 )(OH) 2 , yielding crystal chemically very uncommon topological equivalents of Zn 2+ ≡ S 6+ and Si 4+ ≡ Li + , aside from the expectable substitution Mn 2+ ≡ Co 2+ . Polarised optical absorption spectra of LiCo(SO 4 )OH reveal that the dominating spin-allowed 4 T 1 (P) band system of Co 2+ ( d 7 configuration) is strongly split up and covers a prominent part (~ 15,500–24,500 cm −1 ) of the visible spectral range, in accordance with the significant distortion of the Co(OH) 3 O 3 polyhedron. The spectra are interpreted in terms of the Superposition Model of crystal fields, yielding a new set of intrinsic and interelectronic repulsion parameters for Co 2+ .