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Advances in Condensed Matter Physics
Volume 2013 (2013), Article ID 297504, 7 pages
http://dx.doi.org/10.1155/2013/297504
Research Article

Spectral Investigation of Crystalline (CuHPO4, Cu2P4O12, and Cu2P8O22) and Glassy Copper Phosphates

1Otto-Schott-Institute, Friedrich-Schiller-University Jena, Fraunhoferstraße 6, 07745 Jena, Germany
2Fraunhofer IKTS-Institute for Ceramic Technologies and Systems, Michael-Faraday-Straße 1, 07629 Hermsdorf, Germany
3Skobel’tsyn Research Institute of Nuclear Physics, Moscow State University, Vorob’evy Gory, Moscow 119992, Russia

Received 2 May 2013; Accepted 17 July 2013

Academic Editor: Victor V. Moshchalkov

Copyright © 2013 Christiane Günther et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

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.

1. Introduction

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 [1] and copper pyrophosphate α-Cu2P2O7 [2]. Further condensation in the presence of cations leads to polyphosphates (short chains) or metaphosphates. Two kinds of metaphosphates, the catena phosphates with 2-corner-linked PO4 tetrahedra (infinite chains of Q2 units) and the cyclophosphates (rings of Q2 units), have to be distinguished. In comparison to that, ultraphosphates show linked together Q2 and Q3 (2- and 3-corner-linked PO4 tetrahedra) and build layer or chain structures.

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 [35].

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 [3]. The copper polyphosphates with a molar ratio of Cu/P < 1 are much less numerous, for example, the metaphosphate Cu2P4O12 and the ultraphosphate Cu2P8O22 [4, 69].

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 [10].

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 [1114].

With additional water content, the glasses exhibit changes in structure and properties, like density and colour [1517]. These changes are important for possible applications and need further investigations. According to Bae and Weinberg [10], it is known that by melting, in crucibles a part of the P2O5 evaporates during the melting and in spite of that, P2O5 can absorb a lot of water. Only pure metaphosphate glasses are relatively stable. So, in this work the metaphosphate glass used for the IR-spectroscopy and the ultraphosphate glasses were melted in silica ampoules, which were sealed under vacuum. Partially, definite amounts of water were added for the investigations. The metaphosphate glasses studied with UV-VIS-spectroscopy were melted in a crucible and cooled under different conditions.

2. Experimental

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. [3]. The X-ray diffraction of these crystals was performed on a D5000 of Siemens and is given in Figure 1.

297504.fig.001
Figure 1: 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 [18].

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 1.

tab1
Table 1: List of investigated samples.

3. Results and Discussion

3.1. UV-VIS Spectroscopy

The absorption spectra of the regular octahedrally coordinated Cu2+ ion show one broadband at 12500 cm−1 due to the transition. The Jahn-Teller effect causes a tetragonal distortion of the structure, which leads to the splitting of the orbital energy levels. So, three energy transitions are possible , , and [18].

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 2). In the same range lies the broad Cu2+ band of the octahedrally bonded copper ultraphosphate (Cu2P8O22) [19]. This broad band can be resolved into three-component Gaussian absorption bands at 8900 cm−1, 10200 cm−1, and 12600 cm−1, due to the three energy transitions. One strong band at 10200 cm−1 and weaker ones at lower and higher wavenumbers are refined [1517].

297504.fig.002
Figure 2: 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 2).

In this coordination, two energy transitions are possible and [20]. So, this band was resolved into two-component Gaussian bands centred at 10528 cm−1 and 13233 cm−1, while the second one is slightly stronger (Figure 3).

297504.fig.003
Figure 3: 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 4). Glass 1 was rapidly cooled to the temperature of cold water, and glass 2 was slowly cooled to room temperature (approx. 5 K/min). Thus different kinds of structures were generated. The absorption band of glass 1 is shifted to higher wavenumbers than the band of glass 2, and that of glass 3 is shifted to even lower wavenumbers. So, glass 1 contains the highest part of square-pyramidal coordinated Cu2+.

297504.fig.004
Figure 4: 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 [21]. Thus, the amount of the square-pyramidal coordinated Cu2+ seems to be connected to the amount of Cu1+. Also in all glasses with green colour, certain amounts of water (about 1 mol%) were identified, but the glasses with purposefully added water showed a turquoise colour. Hence, the green colour is connected to water, Cu1+, and a change in the coordination to square-pyramidal coordinated Cu2+.

3.2. IR-Spectroscopy

The IR-spectrum of CuHPO4 exhibits a broad OH band at 3130 cm−1 (Figure 5, Table 2). In the spectrum of Cu2P4O12, a weak OH band is also present. This can be due to adsorbed water at the surface or little amounts of additional water in the structure. The IR-spectrum of Cu2P8O22 does not show an OH band. The OH bands of the glasses cannot be investigated because the glasses are very hygroscopic and adsorb different amounts of water depending on the humidity and the temperature of the environment. The metaphosphate glasses are more resistant than the ultraphosphate glasses, but nonetheless sensitive. Only a very small band in the range between 1400 cm−1 and 1200 cm−1 is registrated and no band in the range between 800 cm−1 and 700 cm−1 in the spectrum of CuHPO4. Thus, no ν(P=O)–, asymmetric ν(PO2)–, and symmetric ν(P–O)-vibrations can be found (Table 2). The structure of this molecule does not allow them, because no pure P=O-groups are present, and to each PO4 tetrahedron, one hydrogen and one copper ions are bound. Only two bridging oxygens are left for each tetrahedron, from which one is connected to the phosphor atom and one is connected to the copper ion of a neighbouring tetrahedron. The short P–O distance between P1–O4 is related to the P–O–Cu bond (O4–Cu: 2,36 Å) and no real double bond [3, 1517].

tab2
Table 2: 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.
297504.fig.005
Figure 5: 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 ν(P–O)-vibrations and the deformation vibrations in the range between 850 cm−1 and 400 cm−1 are very similar in all glasses and can be compared to that ones in Cu2P4O12 and Cu2P8O22 crystals. The most differences in the spectra of the glasses appear in the range between 1400 cm−1 and 850 cm−1 [21]. So, these band complexes were separated with the help of Gaussian fitting (Table 2). In this range, the spectrum of the ultraphosphate glass 3 exhibits a similar trend to the spectrum of the ultraphosphate crystals (Cu2P8O22), although the metaphosphate glass 4 shows similarities to the metaphosphate crystals (Cu2P4O12). Accordingly, the structures of the glasses can be compared to those of the crystals.

3.3. EPR

The EPR spectra could be categorized into four types. The metaphosphate glasses 1 and 2 showed type I spectra (Figure 6). The copper content in these two glasses is very high, and so the spectra show a very broad line with and peak-to-peak width  G for glass 1 (green) and and  G for glass 2 (turquoise). The number of Cu2+ ions contributed to the EPR spectra is about 25% of the total amount of the introduced copper. The rest of the copper might form copper clusters or crystalline units with antiferromagnetic interactions between the Cu2+ ions, which do not contribute to the EPR spectra. The metaphosphate glasses were melted under oxidising atmosphere (air), so the reduction from the Cu2+ of Cu2P4O12 to Cu0 and Cu1+ is very unlikely. Besides Cu0 would change the colour of the glasses to red [22]. The ultraphosphate glasses were melted under vacuum (in sealed ampoules), so the reduction of Cu2+ to Cu1+ is principally possible. But the formation of Cu0 is unlikely, due to the colour of the glasses. Ulraphosphate glasses with a copper concentration in a middle range (between 10 and 30 mol% CuO) exhibit a very narrow single symmetric Lorentzian line of the type II spectra with and  G (Figure 7). The shape and width of this line indicate the exchange interactions among Cu2+ ions in a common spin system. Here, the number of Cu2+ ions contributed to the EPR spectra adds up to 20–30%. The highest number was obtained for a glass containing 10 mol% H2O. Hence, additional water increases the number of Cu2+ ions which contribute to the spectra.

297504.fig.006
Figure 6: EPR spectrum type I (metaphosphate glasses, 50 mol% CuO).
297504.fig.007
Figure 7: EPR spectrum type II (10–30 mol% CuO). Black spectrum experimental, red spectrum calculated.

The structure of spectrum type III (Figure 8) lies between the types I and II. It shows a single line broadened because of spin-spin interactions in a united spin system with higher Cu2+ concentrations. This type occurs for copper ultraphosphate glasses with a higher copper content (>30 mol% CuO). The number of Cu2+ ions contributed to the EPR spectra increases to >45 %. The -factor is with slightly smaller than that for spectrum type II and bigger than that for type I and the peak-to-peak width  G is bigger than that for type II and smaller than that for type I. Subsequently the -factor decreases with increasing the copper content, and the peak-to-peak width decreases.

297504.fig.008
Figure 8: 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 9). Spectrum type 4 is the superposition of two spectra. Both spectra belong to Cu2+, but probably in different coordination states, which could be different kinds of octahedrally distortion or another coordination like square pyramidal.

297504.fig.009
Figure 9: 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 -factor only slightly between and and for the peak-to-peak width between  G and  G. The ratio () of the integrated intensity of the structureless line to that of the HFS spectrum of the last HFS spectrum accounts to 71 : 1 for a turquoise glass without water and to 167 : 1 for a green glass with about 1 mol% water and a certain amount of Cu1+.

The experimental spectra were fitted to simulated ones to determine the spectral parameters of Cu2+ [23, 24]. The simulated spectra were calculated computing spin Hamiltonian of an axial symmetry with electron spin and nuclear spin [25]

The best HFS fit parameters of all samples were  cm−1,  cm−1, , and .

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.

4. Summary

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.

Acknowledgments

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.

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