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 P2 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, wR2 = 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) ₃O ₃ octahedra (< Co–O > = 2.126 Å), which are further linked by common corners, hydrogen bonds, and by properly shaped SO ₄ tetrahedra (< S–O > = 1.476 Å) to sheets parallel (100). These sheets are connected to a three-dimensional framework by sharing corners with distorted LiO ₄ polyhedra (< Li–O > = 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 ₂Mn(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 ⁴T 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) ₃O 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⁺.