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

TL;DR

This study reports the growth of Cu6(Ge,Si)6O18.6H2O single crystals, analyzes their UV-VIS spectra, and compares their electronic excitations with other Cu(II) compounds, revealing relationships between bond strengths and spectral features.

Contribution

It provides a new method for growing Cu6(Ge,Si)6O18.6H2O crystals and offers a detailed spectral assignment and comparison with related compounds, clarifying ligand influences on electronic excitations.

Findings

Successful growth of mm-sized single crystals via diffusion technique.

Identification of spectral features related to Cu(II) ligand environments.

Confirmation of linear relationships between excitation energies and bond strengths.

Abstract

It is reported on growth of mm-sized single-crystals of the low-dimensional S = 1/2 spin compound Cu6(Ge,Si)6O18.6H2O by a diffusion technique in aqueous solution. A route to form Si-rich crystals down to possibly dioptase, the pure silicate, is discussed. Further, the assignment of dd excitations from UV-VIS 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 Jzln(2) is taken into account for cuprates, a single linear relationship between the Dqe excitation energy and equatorial Cu(II)-O bond strength is confirmed for all compounds. A linear representation is also confirmed between 2A1g energies and a function of axial and equatorial Cu-O bond distances, when auxiliary axial bonds are used for four-coordinated compounds. The quotient Dt/Ds of experimental orbital energies deviating from the general trend to smaller values indicates the existence of H2O respectively Cl1- axial ligands in comparison to oxo-ligands, whereas larger Dt/Dqe values indicate missing axial bonds. The quotient of the excitation energy 2A1g by 2x2Eg-2B2g allows to check for correctness of the assignment and to distinguish between axial oxo-ligands and others like H2O or Cl1-. Some assignments previously reported were corrected.