Emerald green crystals of the new composition CuHPO4 were synthesized and compared to (copper metaphosphate) Cu2P4O12 and (copper ultraphosphate) Cu2P8O22. Various copper ultraphosphate and metaphosphate glasses of different compositions between pure P2O5 and Cu2P4O12 were melted in crucibles or in evacuated and sealed silica ampoules at 1473 K or 1373 K for 2 hrs. The UV-VIS-spectra and the IR-spectra of all crystals and glasses were measured and compared. The absorption band of Cu2+ was resolved into more component Gaussian bands and differentiated between square-pyramidal and octahedrally coordinated copper. These structural changes are responsible for colour changes from green to turquoise. The infrared spectroscopic properties of the copper phosphate glasses differ between metaphosphate and ultraphosphate structures but show similarities between the crystals and the glasses of the same structure type. EPR studies of some of the glasses show different kinds of spectra, with increasing copper content.
Orthophosphoric acid condenses to diphosphoric acid under heat treatment. The related salts are called diphosphates (pyrophosphates), which contain P2O7 groups, for example, copper hydroxydiphosphate Cu3[P2O6OH]2 [
Basically, it is differentiated between nonbonded (Q0) monophosphates, primary bonded (Q1) diphosphates, secondary bonded (Q2) meta- and cyclophosphates, and tertiary bonded (Q3) framework phosphates (pure P2O5). Polyphosphates and ultraphosphates are mixed structures (Q1 and Q2, resp., Q2 and Q3) of different bonded structural (PO4) units [
The copper ion in the copper phosphates is regularly coordinated with four, five, or six oxygen atoms, but most structures consist of tetrahedral or octahedral CuO groups. About twenty different structures of copper phosphates have been discovered to date. The larger family is that of copper monophosphates, which shows a molar ratio of Cu/P = 1. The new structure of CuHPO4 does belong to this group [
Phosphate glasses are hygroscopic, but highly resistant against hydrofluoric acid. They can accommodate high concentrations of transition metal ions and remain amorphous. Therefore, invert glasses with more than 50 mol% CuO are possible [
Copper phosphate glasses show very interesting electrical and optical properties. So, they can be used as superionic conductors, heat absorbers, solid-state lasers, colour filters, nonlinear optics, or copper releasing degradable phosphate glass fibres, which have potential uses in wound healing or as plant fertilizers [
With additional water content, the glasses exhibit changes in structure and properties, like density and colour [
Phosphoric acid (85%) and copper oxide were mixed in a mortar. This first reaction took several hours. Afterwards, the mixture was tempered at 373 K for a week and further at 503 K for another two days. Cu2P4O12 with minor rests of Cu2P2O7 was obtained in the form of a light green to turquoise coloured solid. After one week tempering at 373 K, emerald-green needles of the composition CuHPO4 grow, if phosphoric acid (65%) is used. By further tempering with residues of H3PO4, these needles decompose to Cu2P4O12. If the green crystals are kept in the air for several hours, they absorb water, become blue, and decompose to the powder of CuHPO4·H2O. The storage in nonaqueous solvents or under vacuum also leads to decomposition, but this time into turquoise powder. So, the crystals are kept in ethanol. The single crystal X-ray analysis of the new CuHPO4 crystals was published in Günther et al. [
XRD pattern of CuHPo4.
The copper phosphate glasses were prepared of the synthesized Cu2P4O12, P2O5, and partially definite amounts of water. The metaphosphate glasses 1 and 2 were melted in a crucible at 1473 K for 2 hrs and cooled under different conditions. The glasses 3 and 4 were melted in silica ampoules (sealed under vacuum) with slowly cooling. The metaphosphate glass was also melted at 1473 K and the ultraphosphate glasses at 1373 K for 2 h. All those glasses were broken into pieces; the pieces were embedded, cut to plates, and polished for the spectroscopic investigations.
The contents of Cu2+ and Cu1+ of some glasses were determined via complexometric titration [
The microcrystal spectrometer (Fa. Genuine Jackman Parts, Research School of Chemistry, Australian National University) of the University Bonn (working group of Professor, Dr. R. Glaum) provided the UV-VIS-spectrum of CuHPO4. A Shimadzu type UV-3101PC was utilized for the other UV-VIS-spectra.
The infrared spectra of the crystals were measured of powdered samples using the KBr pellet technique, with the help of an IFS 66 Spectrometer of Bruker. The spectra of the glasses were calculated from their reflectance spectra by Kramers-Kronig transformation.
The EPR spectra of some glasses were measured using a spectrometer RE-1306 (Russian model) of X band frequency at 298 K. A list of the investigated samples is given in Table
List of investigated samples.
Powders | ||
1 | CuHPO4 | Copper(II) hydrogen phosphate |
2 | Cu2P4O12 | Copper(II) metaphosphate |
3 | Cu2P8O22 | Copper(II) ultraphosphate |
| ||
Glasses | ||
1 | 50 mol% CuO |
Meta, rapidly cooled, |
2 | 50 mol% CuO |
Meta, slowly cooled, |
3 | 17 mol% CuO + 1 mol% H2O |
Ultra, slowly cooled, |
4 | 50 mol% CuO | Meta, slowly cooled, melted in ampoule |
The absorption spectra of the regular octahedrally coordinated Cu2+ ion show one broadband at 12500 cm−1 due to the
The absorption band of the octahedrally coordinated Cu2+ of the turquoise crystals of copper metaphosphate (Cu2P4O12) is found in the range between 8000 cm−1 and 13000 cm−1 (Figure
Absorbance spectra of CuHPo4 and Cu2P4O12.
The copper of the emerald green crystals of copper hydrogen phosphate (CuHPO4) is square-pyramidal coordinated, and the absorption band is shifted to higher wavenumbers in the range between 9000 cm−1 and 15000 cm−1 (Figure
In this coordination, two energy transitions are possible
Peak fitting of the absorbance spectrum of CuHPo4 (Origin).
The metaphosphate glasses 1 and 2 show shifted maxima and a different form of the Cu2+ band in the range between 7000 cm−1 and 16000 cm−1 (Figure
Absorbance spectra of the metaphosphate glasses 1 and 2 and the ultraphosphate glass 3.
The resolved bands for the octahedral and square-pyramidal coordinated Cu2+ of the crystals were used to create five-component Gaussian absorption bands for the broad Cu2+ band in the glasses. The extinction coefficients of the Cu2+ in the crystals were used to estimate the parts of the different coordinations in the glasses. So, it was discovered that in the metaphosphate glass 1 nearly the half of the copper is square-pyramidal coordinated, in glass 2 nearly one-third, and in the ultraphosphate glass at least a quarter. Therefore, the increase of the copper content leads to more square-pyramidal coordinated copper.
Also the colour in the glasses changed slightly. Glass 1 is green, and glasses 2 and 3 are more turquoise. It is assumed that if the glasses are greener, more copper is square-pyramidal connected. One point in this favour is the structure of the emerald green CuHPO4 crystals. Also rapid cooling leads to more square-pyramidal coordinated Cu2+ than slowly cooling.
Furthermore, it can be assumed that the green glasses contain more Cu1+ (up to 4 mol%) than the turquoise ones, because of the stronger tail of the UV absorption edge which extends into the visible spectrum region [
The IR-spectrum of CuHPO4 exhibits a broad OH band at 3130 cm−1 (Figure
Data of the infrared spectra of CuHPO4, Cu2P4O12, Cu2P8O22, glass 3 (17 mol% CuO + 1 mol% H2O (ultra)), and glass 4 (50 mol% CuO (meta)); data in cm−1; s: strong; m: middle; w: weak, b: broad.
CuHPO4 | Cu2P4O12 | Cu2P8O22 | Glass 3 | Glass 4 | |
---|---|---|---|---|---|
|
3130 m, b | 3410 w, b | — | — | — |
| |||||
|
1214 w | 1325 m |
1360 w |
1396 w |
1375 w |
| |||||
|
1113 m |
1130 w |
1190 w |
1167 w |
1166 w |
| |||||
|
1043 s |
1042 s |
1030 w |
1046 m |
1062 s |
| |||||
|
— | 725 m, b | 760 m |
760 m, b |
750 m, b |
| |||||
Deformation vibration | 611 w |
578 w |
570 w |
470 m, b | 510 m, b |
IR-spectra of the crystals of CuHPo4, Cu2P4O12, Cu2P8O22, the ultraphosphate glass 3, and the metaphosphate glass 4.
The IR-spectra of the crystals of Cu2P4O12 and Cu2P8O22 show a lot of bands in all the ranges between 1400 cm−1 and 400 cm−1. Both structures contain (P=O)-groups and a multitude of different structure elements. The metaphosphate (Cu2P4O12) crystal structure is built up of Q2-bonded rings and the ultraphosphate (Cu2P8O22) crystal structure of Q2- and Q3-bonded layers of 10-membered rings. The additional Q3 bonds lead to even more bands in the spectrum of Cu2P8O22.
The spectra of the glasses exhibit much broader bands than the crystals. The symmetric
The EPR spectra could be categorized into four types. The metaphosphate glasses 1 and 2 showed type I spectra (Figure
EPR spectrum type I (metaphosphate glasses, 50 mol% CuO).
EPR spectrum type II (10–30 mol% CuO). Black spectrum experimental, red spectrum calculated.
The structure of spectrum type III (Figure
EPR spectrum type III (>30 and <50 mol% CuO).
Ultraphosphates with a small copper content (<10 mol% CuO) change the shape of their spectra in comparison to the other glasses (Figure
EPR spectrum type IV (<10 mol% CuO).
One of the spectra exhibits a four-component hyperfine structure (HFS) for parallel orientation and a structureless peak for perpendicular orientation. The second spectrum is Lorentzian single line similar to spectrum type II. The measured data vary for the
The experimental spectra were fitted to simulated ones to determine the spectral parameters of Cu2+ [
The best HFS fit parameters of all samples were
The EPR investigations are much more accurate and higher resolving for small copper contents. Therefore, only for glasses with less than 10 mol% CuO, different Cu2+ coordinations could be detected.
The crystals of CuHPO4, Cu2P4O12, and Cu2P8O22 were compared to copper phosphate glasses. Some of the glasses were prepared in crucibles and the others in evacuated and sealed silica ampoules. The glasses show similar optical properties to the crystals. The Cu2+ band in the UV-VIS-spectra was investigated for square-pyramidal and octahedrally coordinated structures. These structures were compared to the glasses, which show different parts of both coordinations. The colours of the glasses differ from green to turquoise, depending on the coordination, Cu1+ concentration, water content, and cooling procedure of the glasses.
The IR-spectra of the crystals and the glasses can be compared with each other, in spite of the fact that the glasses show much broader bands. The metaphosphate structure is similar in the crystals and the glasses. The same can be observed for the ultraphosphate structure. The most interesting changes from the metaphosphate to the ultraphosphate structure can be found in the range between 1400 cm−1 and 850 cm−1, where the ultraphosphate crystals show much more bands than the metaphosphate crystals. The EPR spectra change with the decreasing copper content, and at least two different coordinated Cu2+ ions were found for glasses with less than 10 mol% CuO.
The authors thank Professor Dr. R. Glaum and V. Dittrich from the Institute of Inorganic Chemistry of the University Bonn, Germany, for permitting the use of their Microcrystal Spectrometer. The authors also thank C. Apfel from the Institute of Inorganic Chemistry of the Friedrich-Schiller-University Jena, Germany, for the X-ray diffraction. Further, the authors thank Dr. A. Kriltz, M. Ludwig, and H. Süß from the Institute of Physical Chemistry of the Friedrich-Schiller-University Jena, Germany, and Drs. M. Müller, S. Ebbinhaus, B. Hartmann and C. Hertig of the Otto-Schott-Institute from the Friedrich-Schiller-University Jena, Germany, for their kind help and work.