TITLE:
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
AUTHORS:
Hans Hermann Otto
KEYWORDS:
Dioptase, Ge-Dioptase, Copper(II) Compounds, Cuprates, Crystal Growth, UV-VIS Spectroscopy, EPR, Color, d-d Excitations, Bond Strength, Super-Exchange Interaction
JOURNAL NAME:
World Journal of Condensed Matter Physics,
Vol.7 No.3,
August
21,
2017
ABSTRACT: Low-dimensional quantum spin
systems with the Cu2+ 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 Cu6(Ge,Si)6O18·6H2O 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 Jz·ln(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 if 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 Cl− axial ligands in comparison to oxo-ligands, whereas larger Dt/Dqe values
indicate missing axial bonds. The quotient of the excitation energy 2A1g by 2·2Eg-2B2g allows checking for correctness of
the assignment and to distinguish between axial oxo-ligands and others like H2O
or Cl−.