Single-crystal Raman spectroscopy of brandholzite Mg[Sb(OH) _6]_2?6H_2O and bottinoite Ni[Sb(OH) _6]_2? 6H_2O and the polycrystalline Raman spectrum of mopungite Na[Sb(OH)_6]

摘要

The single-crystal Raman spectra of minerals brandholzite and bottinoite, formula M[Sb(OH)_6]_2?6H_2O, where M is Mg~(+2) and Ni~(+20, respectively, and the non-aligned Raman spectrum of mopungite, formula Na[Sb(OH)_6], are presented for the first time. The mixed metal minerals comprise alternating layers of [Sb(OH) _6]~(-1) octahedra and mixed [M(H_2O) _6]~(+2)/[Sb(OH)_6]~(-1) octahedra. Mopungite comprises hydrogen-bonded layers of [Sb(OH)_6]~(-1) octahedra linked within the layer by Na~+ ions. The spectra of the three minerals were dominated by the Sb-O symmetric stretch of the [Sb(OH) _6]~(-1) octahedron, which occurs at approximately 620 cm ~(-1). The Raman spectrum of mopungite showed many similarities to spectra of the di-octahedral minerals, supporting the view that the Sb octahedra give rise to most of the Raman bands observed, particularly below 1200 cm ~(-1). Assignments have been proposed on the basis of the spectral comparison between the minerals, prior literature and density functional theory (DFT) calculations of the vibrational spectra of the free [Sb(OH) _6]~(-1) and [M(H_2O)_6]~(+2) octahedra by a model chemistry of B3LYP/6-31G(d) and lanl2dz for the Sb atom. The single-crystal spectra showed good mode separation, allowing most of the bands to be assigned to the symmetry species A or E.