# 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.

## Key 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.

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Source: https://tomesphere.com/paper/1705.00526