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) 6O 18 ·6H 2O 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 Dq 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 Dt/ Ds of experimental orbital energies deviating from the general trend to smaller values indicates the existence of H 2O respectively Cl − axial ligands in comparison to oxo-ligands, whereas larger Dt/ Dq 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 2O or Cl −.
Low-dimensional quantum spin systems are of considerable theoretical and experimental interests together with some applications to which they may lead. In spite of the ability of the d9 transition metal ion Cu2+ to form, apart from 3D networks, chains, ladders and small clusters, copper compounds are among the most interesting phases. With equal electronegativity compared to silicon, but in contrast to its tetrahedral networks, Cu(II) mainly forms oxo-compounds with chains and networks of connected “octahedra”.
For instance, copper polygermanate, CuGeO3, has a rather simple crystal structure of “einer” single chains of GeO4 tetrahedra alongside S = 1/2 spin single chains of edge-sharing CuO4+2 octahedra [
The rhombohedral title compound Cu6(Ge,Si)6O18・6H2O represents a hexacyclo-germanate (silicate) that contains copper-oxygen spiral chains along the c-axis, which are connected (intra-chain) by edge-sharing dimers (
This structure is interesting because it allows for a quantum phase transition between an anti-ferromagnetically ordered state and a quantum spin liquid [
If near the empty structural channels located water molecules are removed, a screwed framework of edge-sharing disphenoids rather than flat CuO4 plaquettes remains in the dehydrated compound.
As part of a systematic study of transition metal germanates, silicates and arsenates we have undertaken syntheses of rare copper minerals and new copper compounds in view of its power as low dimensional S = 1/2 spin compounds allowing for interesting physical and physicochemical properties. First, the synthesis serves not to waste rare mineral specimens for research. There is also the possibility to study an improvement in the crystal growth by replacement of copper by other elements, apart from the chance of doping with electronically or magnetically interesting ones. For example, the replacement of copper by manganese was observed in natural samples of dioptase by EPR measurements [
Because the assignment of the dd excitations derived from the UV-VIS spectra of copper-bearing compounds are often found to be incorrect, this work contributed some simple tools that could lead to the right assignment. It is not the intent of this paper to review UV-VIS spectroscopy of Cu2+ compounds in general.
The method described below was used by the author many years earlier for the synthesis of rare minerals, for instance, the synthesis of Pb3Ge(OH)6(SO4)2・3H2O, the piezoelectric Tsumeb mineral fleischerite [
The desired slow diffusion of the distinct solutions into one another leads to the formation of Cu6(Ge,Si)6O18・6H2O seeds that grow up to 1 mm size of light blue crystals within 8 weeks. Interestingly, most individual crystals form double- crystals. The symmetry situation of this finding must be investigated further. The crystals of stocky prismatic, nearly spherical habit developed {110} and {021} forms (
One can extrapolate the time scale to get a crystal of about 2 mm diameter and calculate about 1 year of growing time. Trying to exchange Ge by Si by this method
seems to be less efficient, only a slightly greenish sheen shows that a small exchange occurred.
The other method of co-precipitation of GeO2, SiO2 and Cu(OH)2 gel and longer time vigorous stirring resulted in a vivid green colored polycrystalline material of about 12 at-% Si determined from lattice parameter changes [
A proposed approach for a possible synthesis of pure polycrystalline dioptase results as follows. The first step will be the spontaneous formation of pure germanate and exchange of maximum Ge by Si through stirring or sonochemical treatment. Then pH, as well as temperature, is altered to increase the solubility of the still Ge-rich compound combined with a simultaneous offer of more Si to form a dioptase layer. A new core of silico-germanate can be grown epitaxially and subsequently transformed to dioptase. Repetition of this process may finally form pure dioptase in mm-sized crystals. An automated process would make sense. Nature has similar tools in the quiver such as rhythmic property changes (concentration, pH, temperature) of metal bearing ascending or descending solutions, apart from a lot of time.
A single-phase crystalline powder of synthetic Ge-dioptase for the UV-VIS spectroscopic investigation is best obtained from an aqueous solution of pH 5.5 at room temperature, formed by mixing and stirring equal amounts of 0.02 M cupric acetate with freshly produced 0.02 M GeO2 solution. The initially formed gel settles as fully crystalline precipitate after an induction period of two days [
Complete dehydration of synthetic Ge-dioptase was performed by annealing of the polycrystalline sample up to 920 K for 6 h, followed by cooling down to room temperature with a moderate cooling rate of 20 K/h. The chosen annealing temperature lies about 53 K below the temperature of decomposition to the orthorhombic spin-Peierls phase CuGeO3 [
A natural dioptase samples from the locality Altyn Tyube, Kazakhstan, was used as pure silicate sample. Its complete dehydration to “black” dioptase occurs at 660 K and should be controlled by X-ray powder diffraction analysis because decomposition into CuO (tenorite) and SiO2 (partly quartz and cristobalite) starts only a few degrees higher at 673 K.
First results of UV-VIS spectroscopy on Cu6(Ge,Si)6O18・6H2O are given in the doctoral theses of my coworkers Brandt [
Brandt [
A reinvestigation of the fully hydrated and dehydrated compounds is primarily undertaken in order to deconvolute and understand the broad UV-VIS spectrum of the synthetic color pigment litidionite, KNaCuSi4O10 [
The room temperature UV-VIS spectra of the samples were taken with the double-beam light scanning UV-2501PC CE spectrometer from Shimazu with selectable light sources (50W halogen lamp and D2 lamp). The powder sample was coated on a polished aluminum disk and measured in the reflection modus against a BaSO4 standard in the wavelength range between 190 and 900 nm with a spectral bandwidth of 0.1 nm using a 50 nm/min scan and choosing 0.5 nm intervals. From the less structured absorbance profile, recalculated from the measured reflectance, the energy bands were fitted with Gaussian profile functions. The better resolved spectra of the dehydrated compounds were fitted first and then the results used as start parameters for the broad spectra of the hydrated compounds.
Electron paramagnetic resonance spectroscopy (EPR) provides information about the electronic structure of transition metal ion complexes. For d1,9 systems such as Cu2+ centered complexes with no fine structure the principal values of the g-tensor of the spin Hamiltonian H = βeB・g・S, reflecting the symmetry of the ligand field, can be derived from the EPR spectrum, where B is the external magnetic field, S is the spin vector, and βe = ge・μB (Landé g-factor for the free electron, ge = 2.0023, Bohr magneton μB). In this contribution g values for dioptase, Cu6Si6O18・6H2O, determined by Reddy et al. [
The Gaussian peak analysis of the UV-VIS spectra was performed with the aid of own Turbo-Basic programs using recast software modules once developed for X-ray powder profile analysis, supplemented by a program to provide an illustration of single Gaussian peaks besides the cumulative curve. Fortunately, the spectra of the dehydrated compounds are well-resolved and their reliably fitted profile data could serve as input for the less-resolved spectra of the as-grown respectively hydrated natural compounds, thereby applying variable constraints to parameters (mainly the band width) during successive refinement cycles. Results of a Gaussian deconvolution of the UV-VIS spectra for the hydrated and dehydrated compounds, respectively, are given in
Cu6Si6O18・6H2O (dioptase) | Cu6Ge6O18・6H2O (Ge-dioptase) | ||||||||
---|---|---|---|---|---|---|---|---|---|
P | λ(nm) | Γ(nm) | E(cm−1) | Assignment | P | λ(nm) | Γ(nm) | E(cm−1) | Assignment |
329 | 935 ± 37 | 208 | 10,700 | ΔA | 1000 | 842 ± 30 | 198 | 11,884 | ΔB + ΔA |
815 | 869 ± 12 | 208 | 11,507 | ΔB | |||||
1000 | 695 ± 7 | 194 | 14,400 | ΔE | 645 | 698 ± 24 | 166 | 14,321 | ΔE |
256 | 600 | 109 | 16,670 | ZRS? | 185 | 616 | 129 | 16,230 | ZRS? |
421 | 414 | 108 | 24,160 | SPE? | 271 | 462 | 147 | 21,650 | SPE? |
(554) | 326 | 102 | 30,660 | Eg | (531) | 353 | 97 | 28,350 | Eg |
Cu6Si6O18 (dioptase dehydrated) | Cu6Ge6O18 (Ge-dioptase dehydrated) | ||||||||
P | λ(nm) | Γ(nm) | E(cm−1) | Assignment | P | λ (nm) | Γ(nm) | E(cm−1) | Assignment |
1000 | 811 ± 14 | 158 | 12,330 | ΔB | 1000 | 827 ± 14 | 155 | 12,100 | ΔB |
866 | 668 ± 12 | 120 | 14,960 | ΔE | 843 | 679 ± 12 | 127 | 14,723 | ΔE |
492 | 558 ± 11 | 96 | 17,930 | ΔA | 450 | 565 ± 12 | 97 | 17,700 | ΔA |
235 | 441 | 100 | 22,680 | SPE? | 355 | 437 | 108 | 22,880 | SPE? |
(869) | 330 | 120 | 30,300 | Eg | (630) | 357 | 102 | 28,000 | Eg |
P integrated band intensity (arbitrary units), Γ(nm) full band width at half f, E(cm−1) band energy, SPE assumed simultaneous pair excitation, ZRS less intense band observed only in the hydrated compounds around 2 eV could correspond to a Zhang-Rice singlet excitation, Eg large energy gap.
remarkable integrated band intensity P (given in arbitrary units) is the consequence of non-zero dd transition probabilities due to the absence of symmetry elements on the Cu position with C1 site symmetry and the disphenoidic (stocky tetrahedral) oxygen environment with 4 distinct equatorial bond lengths indicating Cu3d-O2p hybridization. The relative width Γ/λ of the bands of the dehydrated compounds is about 18%, whereas that of the hydrated ones suffer additional broadening to about 23% caused by a vibronic contribution of the water molecule rings and due to assumed peak overlapping according to the below presented assignment.
The steep increase of absorption at the badly resolved high energy limit of the UV-VIS spectra has been simulated by a Gaussian curve, too, and may be interpreted as absorption edge, the large gap between valence and conduction band of isolator compounds. The gap is determined around 3.80 eV for dioptase and shifts to 3.76 eV for Ge-dioptase, respectively. It is slightly lower for the dehydrated compounds, giving 3.52 and 3.47 eV, respectively (
The color of Cu2+ compounds with their Jahn-Teller distorted coordination polyhedra [
The transition energies ∆n (cm−1), derived from broad Gaussian-shaped absorption bands of UV-VIS spectra, are the energy differences between the
Conversely, the D parameters can be recalculated as
Whereas ∆E is always moderately larger than ∆B, ∆A ranges from about 8500 cm−1 (<∆B) for shortest axial bonds to at least 21,500 cm−1 (>∆E) for axially non- existent bonds (squared-planar coordination).
A more quantitative description of ligand field parameters using effective charges and bond lengths results in the following relations [
where
Quoting Gerloch and Slade [
For comparison of calculated band energies with experimental ones given in cm−1 an energy conversion factor
Lebernegg et al. [
The plot is depicted in
We chose compounds of the Egyptian Blue family (cuprorivaite, wesselite, effenbergite,) with isolated D4h plaquettes, the dehydrated dioptase compounds with equatorially edge-shared dimers, further connected via water oxygen to
corner-shared spiral chains in the fully hydrated compounds, litidionite as characterized by pyramid-edge-shared dimers (cis-arrangement), in contrast to lammerite with infinite chains of such units and with two distinct Cu sites, further azurite with “octahedral” chains (two distinct sites), and finally conichalcite and CuGeO3 showing infinite single chains with equatorially edge-shared “octahedra”. One may learn more about the structural hierarchy of special copper oxy-salt minerals from Eby and Hawthorne [
Compound | CN | d(Cu-O) (Å) | Bond strength | Reference | ||
---|---|---|---|---|---|---|
se | sa | Ss | ||||
Ca0.5Sr0.5CuO4 | 4 | 1.945 | 1.913 | - | 1.913 | [ |
CaCuO2 | 1.928 | 2.004 | - | 2.004 | [ | |
BaCuSi4O10 Effenbergite | 1.925 | 2.022 | - | 2.022 | [ | |
SrCuSi4O10 Wesselite | 1.925 | 2.022 | - | 2.022 | [ | |
CaCuSi4O10 Cuprorivaite | 1.929 | 1.998 | - | 1.998 | [ | |
Cu6Si6O18 | 4 + (1) | 1.9250 1.9294 1.9354 1.9466 3.3153 | 1.969 | 0.023 | 1.992 | [ |
Cu6Ge6O18 | 1.9043 1.9284 1.9380 1.9979 3.3841 | 1.954 | - | 1.954 | [ | |
Y2BaCuO5 | 2 + 2 + 1 | 1.985 1.988 2.206 | 1.692 | 0.232 | 1.923 | [ |
CuGeO3 | 4 + 2 | 1.941 2.926 | 1.941 | 0.093 | 2.022 | [ |
Cu6Si6O18・6H2O | 6 | 1.952 1.952 1.959 1.983 2.502 2.648 | 1.818 | 0.195 | 2.014 | [ |
Cu6Ge5.4Si0.6O18・6H2O | 6 | [ | ||||
Cu6Ge6O18・6H2O | 6 | 1.9037 1.9486 1.9547 1.9884 2.6364 2.6696 | 1.894 | 0.159 | 2.053 | [ |
Cu3(CO3)2(OH)2 Azurite | 2 + 2 + 2 Cu(1) site | 1.9387 1.9455 2.9840 | 1.953 | 0.083 | 2.036 | [ |
6 Cu(2) site | 1.9385 1.9388 1.9675 1.9947 2.3608 2.7578 | 1.830 | 0.223 | 2.053 | ||
CuSO4 ・5H2O Chalcanthite | 2+2+2 Cu(1) site | 1.9748 1.9770 2.3858 | 1.769 | 0.250 | 2.019 | [ |
2+2+2 Cu(2) site | 1.9447 1.9696 2.4400 | 1.739 | 0.293 | 2.033 | ||
KNaCuSi4O10 Litidionite | 6 | 1.9220 1.9434 1.9683 1.9799 2.6238 3.4024 | 1.863 | 0.109 | 1.972 | [ |
Cu3(AsO4)2 Lammerite | 2 + 2 + 2 Cu(1) site | 1.933 1.974 2.923 | 1.864 | 0.093 | 1.957 | [ |
6 Cu(2) site | 1.941 1.947 1.972 2.028 2.282 2.782 | 1.772 | 0.254 | 2.026 | ||
CaCuAsO4OH Conichalcite | 6 | 1.8850 1.8855 2.0666 2.0688 2.2976 2.3882 | 1.811 | 0.332 | 2.143 | [ |
La2CuO4 | 4 + 2 | 1.9043 2.4145 | 2.151 | 0.278 | 2.439 | [ |
Sr2CuO2Cl2 | 4 + 2 | 1.9864 2.860 | 1.687 | 0.292 | 1.979 | [ |
Compound | Color | Cu site symmetry | ΔB | ΔE | ΔA | Other Transition | Reference |
---|---|---|---|---|---|---|---|
CaCuO2 (infinite layer) | dark-grey | D4h | 13,230 | 15,730 | 21,370 | - | [ |
Tenorite CuO | brownish | Ci | 12,170 | 12,930 15,530 | 16,670 | many | [ |
Conichalcite synth. CaCuAsO4OH | light green | C1 | 10,575 | 12,500 | 8585 | 15,313 ZRS? 31,370 (Eg) | [ |
Lammerite synth. Cu3(AsO4)2 | dark green | C1 Ci | 11,530 | 12,400 14,050 | 10,200 14,050 | 23,260 CuO 31,250 (Eg) | [ |
Y2BaCuO5 | vivid green | D4h | 12,500 | 14,700 | 25,970 CT | [ | |
10,700 | 13,200 | ||||||
Azurite Cu3(CO3)2(OH)2 | blue | Ci C1 | 11,806 11,806 | 16,484 11,806 | 16,484 11,806 | 19,793 17,952 | [ |
Chalcanthite CuSO4・5H2O | deep blue | C1 C1 | 11,407 11,860 | 13,308 13,488 | 9699 9735 | 15,234 ZRS? 15,567 | [ |
Litidionite synth. KNaCuSi4O10 | light blue | Ci | 11,723 | 14,700 | 13,900 | 21,400 SPE 31,600 (Eg) | [ |
Cu6Ge6O18 | dirty blue | C1 | 12,100 | 14,723 | 17,700 | 22,880 SPE | [ |
Cu6Ge6O18・6H2O | bluish-green | C1 | 11,880 | 14,320 | 11,880 | 16,230 ZRS? 21,650 CT 30,300 (Eg) | [ |
Cu6Ge5.4Si0.6O18・6H2O | dark green | C1 | 13,300 | 19,400 | 23,900 CT | [ | |
CuGeO3 | turquoise | D2h | 12,570 | 13,970 | 12,920 | 15,733 ZRS? 15,800 | [ |
BaCuSi4O10 synth. (Effenbergite) | deep blue | D4h | 12,200 | 15,950 | 18,520 | - | [ |
SrCuSi4O10 synth. (Wesselite) | 12,480 | 16,050 | 18,520 | - | [ | ||
CaCuSi4O10 synth. (Cuprorivaite) | 12,590 12,740 | 15760 16130 | 18,530 18,520 | - | [ | ||
Dioptase dehydrated Dioptase partly dehydrated | black dark blue | C1 | 12,330 12,500 | 14,960 14,500 | 17,930 17,600 | 22,680 SPE - | This work; [ |
Dioptase Cu6Si6O18・6H2O | emerald green | C1 | 11,500 12,495 | 14,500 15,010 | 17,000 10,200 | [ | |
11,507 | 14,400 | 10,700 | 16,670 ZRS? 24,160 CT 30,660 (Eg) | This work |
*synthetic lammerite with an amount of CuO; **for this work a different assignment as given in the reference was used.
Recently, the energy and symmetry of dd excitations of some undoped layered cuprates have been measured by CuL3 resonant X-ray scattering [
The different connectedness of cuprates in comparison to the dioptase group is manifested in a larger contribution of the principal magnetic super-exchange interaction Jz to the optical excitation energies. In
with an exponent near 5/3, explained by chemical pressure (Rocquefelte et al., 2012) [
The R−5 inverse power of Cu-O bond lengths is nearly a measure for the bond strength. Therefore, the reliability of the fit can be enhanced applying the empirical Cu-O bond strength relation s = Σ(R/R0)−N [
bond strength sum se of the four equatorial bonds. New values R0 = 1.713(9) Å, N = 5.76(16) were re-calculated for this work [
respectively
for both the dioptase group and cuprates. ΔB of the last mentioned group is corrected by a bond angle dependent (magnetic) contribution (
Turning now to the calculation of splitting parameters Ds (Equation (9)) and Dt (Equation (10)) involving axial ligands one has to distinguish according to the Nephelauxetic Effect between pure oxo-ligands and such ones as H2O or Cl− with increased cationic charge and assumed higher Dqa values [
In the case of square-planar environment it is useful to limit the extent of the
In order to check the correct assignment one can use a relation between experimental B2g, Eg and A1g values of form
with a Ra/Re ratio including well adapted auxiliary Ra bonds for compounds of coordination number 4, but different ε values for the dioptase group (
It should be stressed with respect to the use of mean bond distances in Equation (13) that also in the Equations (7) to (10) the mean of corresponding bond distances is taken first and then their inverse fifth power is calculated to yield the convincing results of
An additional scaling
Indeed, the connectedness of copper-ligand units, representing the number of shared copper-oxygen polyhedra, should be important for the dd excitation energy. Therefore, besides the equatorial ligand sums that are calculated as fit coordinate, we used the bond valence sums to check for inconsistent structural details and signs for mixed valences. Copper polygermanate in the Pbmm prototypic structure [
Isolated CuO plaquettes, clusters and chains: Calculation with | ||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Phase | ΔB | ΔE | ΔA | Ds | Dq | Dt | Dt/Dq | Dt/Ds | f(Δ) | f(Δ)/f(R) | Ligands | |||
equat. | axial | |||||||||||||
Cuprorivaite | 12,590 12,501 | 15,760 15,446 | 18,530 18,414 | 3100 3051 | 1259 1250 | 1226 1241 | 0.974 | 0.395 | 0.979 | 0.956 | oxygen | no | ||
CuGeO3 | 12,570 12,439 | 13,970 14,209 | 13,970 14,324 | 2196 2299 | 1257 1244 | 1037 1026 | 0.825 | 0.472 | 0.909 | 1.081 | oxygen | oxygen | ||
Effenbergite | 12,500 12,566 | 15,950 15,895 | 18,520 18,591 | 3139 3132 | 1250 1257 | 1139 1213 | 0.955 | 0.380 | 0.955 | 0.930 | oxygen | no | ||
Wesselite | 12,480 12,566 | 16,050 15,895 | 18,520 18,591 | 3156 3132 | 1248 1257 | 1179 1213 | 0.945 | 0.374 | 0.944 | 0.920 | oxygen | no | ||
Dioptase black | 12,330 12,324 | 14,960 15,057 | 17,930 17,850 | 2937 2940 | 1233 1232 | 1236 1218 | 1.003 | 0.421 | 1.019 | 1.013 | oxygen | no | ||
Cu6Ge6O18 | 12,100 12,123 | 14,723 14,845 | 17,700 17,606 | 2903 2904 | 1210 1212 | 1217 1198 | 1.006 | 0.419 | 1.020 | 1.017 | oxygen | no | ||
Ge-Dioptase | 11,884 11,929 | 14,321 14,434 | 11,884 12,084 | 2046 2084 | 1188 1193 | 740 750 | 0.623 | 0.362 | 0.709 | 0.866 | oxygen | H2O | ||
Azurite (1) | 11,806 12,070 | 16,484 14,162 | 16,484 15,014 | − 2444 | 1181 1207 | − 1048 | 0.770 | 0.317 | 0.809 | 1.024 | OH−/ oxygen | oxygen | ||
Azurite (2) | 11,550 11,589 | 12,770 12,924 | 11,300 11,669 | 1789 1858 | 1155 1159 | 829 848 | 0.718 | 0.464 | 0.808 | 1.067 | OH−/ oxygen | oxygen | ||
Litidionite | 11,723 11,794 | 14,700 14,465 | 13,900 13,917 | 2411 2397 | 1172 1179 | 851 866 | 0.726 | 0.353 | 0.786 | 0.915 | oxygen | oxygen | ||
Lammerite (1) | 11,780 11,735 | 13,744 13,744 | 14,356 14,357 | 2331 2328 | 1178 1174 | 1006 1000 | 0.854 | 0.432 | 0.914 | 0.914 | oxygen | oxygen | ||
Lammerite (2) | 11,280 11,231 | 12,400 12,425 | 10,200 10,279 | 1617 1639 | 1128 1123 | 746 745 | 0.662 | 0.461 | 0.754 | 1.023 | oxygen | oxygen | ||
Chalcanthite (1) | 11,407 11,135 | 12,600 12,302 | 8900 8828 | 1442 1428 | 1141 1113 | 627 623 | 0.549 | 0.435 | 0.645 | 0.906 | H2O | oxygen | ||
Chalcanthite (2) | 11,860 11,678 | 13,488 13,369 | 9735 98,108 | 1623 1643 | 1186 1168 | 648 648 | 0.547 | 0.399 | 0.644 | 0.875 | H2O | oxygen | ||
Dioptase green | 11,508 11,491 | 14,398 14,094 | 10,700 10,325 | 1940 1847 | 1151 1149 | 588 587 | 0.511 | 0.303 | 0.619 | 0.801 | oxygen | H2O | ||
Conichalcite | 11,400 11,119 | 12,195 11,922 | 8585 8449 | 1340 1339 | 1140 1141 | 645 658 | 0.566 | 0.481 | 0.661 | 0.940 | OH− | oxygen | ||
Y2BaCuO5 | 10,700 13,200*) | 14,700 14,571 | 2457 2435 | 1070 1073 | 974 966 | 0.911 | 0.397 | 0.936 | 0.904 | oxygen | oxygen | |||
10,732 | 13,205 | |||||||||||||
Cuprates (undoped): Calculation with | ||||||||||||||
Phase | ΔB | ΔE | ΔA | Ds | Dq | Dt | Dt/Dq | Dt/Ds | f(Δ) | f(Δ)/f(R) | Ligands | |||
equat. | axial | |||||||||||||
La2CuO4 | 14,516 14,308 | 17,097 16,833 | 13,710 13,664 | 2327 2313 | 1452 1431 | 880 883 | 0.606 | 0.378 | 0.697 | 0.788 | oxygen | O−? | ||
CaCuO2 | 13,226 13,322 | 15,726 15,982 | 21,370 21,671 | 3410 3476 | 1323 1332 | 1546 1554 | 1.169 | 0.453 | 1.173 | 0.994 | oxygen | no | ||
Sr0.5Ca0.5CuO2 | 12,581 12,744 | 15,565 15,494 | 21,452 21,026 | 3491 3397 | 1258 1274 | 1498 1488 | 1.190 | 0.429 | 1.157 | 0.976 | oxygen | no | ||
NdBa2Cu3O6 | 12,258 12,372 | 14,113 13,741 | 15,968 15,940 | 2546 2473 | 1226 1237 | 1157 1210 | 0.944 | 0.454 | 1.000 | 1.072 | oxygen | oxygen | ||
CuO (tenorite) | 12,170 12,254 | 14,230 14,520 | 16,670 16,878 | 2676 2735 | 1217 1225 | 1193 1188 | 0.981 | 0.446 | 1.023 | 1.027 | oxygen | oxygen | ||
Sr2CuO2Cl2 | 12,097 11,838 | 14,839 14,756 | 15,887 16,021 | 2661 2706 | 1210 1184 | 1048 1040 | 0.867 | 0.394 | 0.904 | 0.897 | oxygen | Cl− | ||
*) The broad band at 12,500 cm−1 is proven to split into two bands at about 10,700 cm−1 and 13,200 cm−1, respectively.
Phase | CN | e> (Å) | a> (Å) | ΔA (exp.) | ΔA (calc.) |
---|---|---|---|---|---|
Cuprorivaite | [ | 1.9307 | 3.35 | 18,530 | 18,393 |
CuGeO3 | [4 + 2] | 1.9326 | 2.7549 | 12,920 | 12,854 |
Effenbergite | [ | 1.9265 | 3.35 | 18,520 | 18,460 |
Wesselite | [ | 1.9265 | 3.35 | 18,520 | 18,460 |
Dioptase black | [ | 1.9340 | 3.30 | 17,930 | 17,874 |
Cu6Ge6O18 | [ | 1.9404 | 3.30 | 17,700 | 17,777 |
Ge-Dioptase | [4 + 2] | 1.9474 | 2.6622 | 11,884 | 11,808 |
Azurite (1) | [4 + 2] | 1.9434 | 2.9840 | 16,488 (?) | 14,821 |
Azurite (2) | [4 + 1 + 1] | 1.9593 | 2.5143 | 11,806 | 10,309 |
Litidionite | [4 + 2] | 1.9525 | 2.8434 | 13,900 | 13,404 |
Lammerite (1) | [4 + 2] | 1.9529 | 2.9230 | 14,350 | 14,128 |
Lammerite (2) | [4 + 1 + 1] | 1.9703 | 2.4609 | 10,200 | 9695 |
Chalcanthite (1) | [4 + 2] | 1.9759 | 2.3858 | 8900 | 8952 |
Chalcanthite (2) | [4 + 2] | 1.9569 | 2.4400 | 9735 | 9657 |
Dioptase green | [4 + 2] | 1.9613 | 2.5688 | 10,700 | 10,735 |
Conichalcite | [2 + 2 + 2] | 1.9640 | 2.3403 | 8585 | 8668 |
Y2BaCuO5 | [4 + 1] | 1.9899 | 2.196 + 3.90 | 14,700 | 14,754 |
La2CuO4 | [4 + 2] | 1.9043 | 2.4045 | 13,710 | 13,646 |
CaCuO2 | [ | 1.9281 | 3.26 | 21,370 | 21,306 |
Sr0.5Ca0.5CuO2 | [ | 1.9440 | 3.30 | 21,452 | 21,426 |
NdBa2Cu3O7-δ | [4 + 1] | 1.9609 | 2.275 + 3.25 | 15,968 | 16,260 |
CuO (tenorite) | [4 + 2] | 1.9558 | 2.7842 | 16,670 | 16,524 |
Sr2CuO2Cl2 | [4 + 2] | 1.9864 | 2.8600 | 15,887 | 16,841 |
strength is reduced towards the net charge of 2+. Even large thermal displacement ellipsoids indicate structural features that require a careful evaluation. Bond lengths should be corrected for “thermal” displacement because not less than their inverse fifth power is used in calculations (see for instance [
Finally, the assignment of the dd excitations can be compared with results of EPR measurements. For 3d9 ions in (nearly) tetragonal ligand symmetry one can apply the following two formulas for the principal components g|| and g﬩, if the ground state is 2B1g:
where ge = 2.0023 is the g-value for the free electron, and λ is the spin-orbital coupling parameter, which yields for the free Cu2+ ion λo = 829 cm−1 [
The k values are the spin orbital reduction factors used to scale the coupling parameters to the free Cu2+ ion value, k = λ/λo. This parameter reduction is attributed to covalence effects.
Notation | Dioptase | Ge-Dioptase |
---|---|---|
ΔB | 11,508 | 11,884 |
ΔE | 14,395 | 14,321 |
ΔA | 10,700 | 11,884 |
g|| | 2.3601 | 2.3780 |
g﬩ | 2.0511 | 2.0970 |
λ|| | −504.08 | −545.53 |
k|| | 0.608 | 0.658 |
λ ﬩ | −346.91 | −662.63 |
k﬩ | 0.418 | 0.799 |
As shown, a comparative reappraisal of Cu2+ UV-VIS spectra benefits from a special consideration of crystal-chemically similar groups of compounds, com- paring exemplarily the dioptase group, covering minerals as well as synthetic samples, with cuprates. The assignment of dd excitations and their representa- tion each on a single curve is possible by attributing a magnetic (bond angle de- pendent) contribution to the cuprate group. It is recommended to extend the bond strength-bond length relation by a bond angle dependent (magnetic) con- tribution. Deviations of the linear representation of orbital excitation energies may be helpful to discriminate results of compounds with peculiar orbital features from those with normal behavior. Fortunately, the first done assignment of well-resolved spectra of dehydrated dioptase Cu6(Ge,Si)6O18 served as input data to deconvolute the badly resolved spectra of as-grown Cu6(Ge,Si)6O18・6H2O samples. At present, the deconvolution of superposed spectra resulting from different Cu sites of a structure is inadequate. However, a pre-calculation of the expected energy levels can serve as input for fitting the experimental spectra. This has been successfully applied to lammerite. It is recommended to take a series of UV-VIS spectra step by step over the entire temperature range from hydrated to fully dehydrated dioptase as a didactic tool to follow the energy levels and their correct assignment, thereby simultaneously controlling the crystal water content by IR spectroscopy with a device that offers both analytical possibilities. Especially it should be investigated whether a Zhang-Rice excitation like that observed for CuGeO3 can be confirmed for the hydrated compounds of the dioptase family, too. In addition, the proposed assignment of the dd excitations of the green phase YBa2CuO5 should be supported by a CuL3 resonant X-ray scattering investigation.
The author would like to thank colleague Prof. Bernd Lehmann for supporting this work by the donation of wonderful dioptase pieces from Altyn-Tyube, Kazakhstan. Also my teacher, the late Prof. Hugo Strunz, donated dioptase pieces from the Tsumeb mine, Namibia.
The author declares no conflict of interest.
Otto, H.H. (2017) 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. World Journal of Condensed Matter Physics, 7, 57-79. https://doi.org/10.4236/wjcmp.2017.73006
Scaling factor notation | Cuprates | Dioptase group | Ratio | |
---|---|---|---|---|
1.755 | 1.733 | 1.013 | ||
0.936 | 0.761 | 1.230 | 1.243 | |
1.587 | 1.276 | 1.244 | ||
2.180 | 1.738 | 1.254 |