Crystal Growth of Cu6(Ge,Si)6O18·6H2O and Assignment of UV-VIS Spectra in Comparison to Dehydrated Dioptase and Selected Cu(II) Oxo-Compounds Including Cuprates

Abstract

Low-dimensional quantum spin systems with the Cu 2+ central ion are still in the focus of experimental and theoretical research. Here is reported on growth of mm-sized single-crystals of the low-dimensional S = 1/2 spin compound Cu 6 (Ge,Si) 6 O 18 · 6H 2 O by a diffusion technique in aqueous solution. A route to form Si-rich crystals down to possible dioptase, the pure silicate, is discussed. Motivated by previously reported incorrect assignments of UV-VIS spectra, the assignment of dd excitations from such spectra of the hexahydrate and the fully dehydrated compound is proposed in comparison to dioptase and selected Cu(II) oxo-compounds using bond strength considerations. Non-doped cuprates as layer compounds show higher excitation energies than the title compound. However, when the antiferromagnetic interaction energy as J z · ln(2) is taken into account for cuprates, a single linear relationship between the D q e excitation energy and equatorial Cu(II)-O bond strength is confirmed for all compounds. A linear representation is also confirmed between 2 A 1g energies and a function of axial and equatorial Cu-O bond distances if auxiliary axial bonds are used for four-coordinated compounds. The quotient D t/ D s of experimental orbital energies deviating from the general trend to smaller values indicates the existence of H 2 O respectively Cl − axial ligands in comparison to oxo-ligands, whereas larger D t/ D q e values indicate missing axial bonds. The quotient of the excitation energy 2 A 1g by 2 · 2 E g - 2 B 2g allows checking for correctness of the assignment and to distinguish between axial oxo-ligands and others like H 2 O or Cl − .