• Copper;
  • Indium;
  • Phosphate;
  • Single-crystal structure;
  • Phase diagrams;
  • Solid state reaction;
  • Thermal analysis


  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Section
  5. Results and Discussion
  6. Conclusions

The system CuO/In2O3/P2O5 has been investigated using solid state reaction between CuO, In2O3 and (NH4)2HPO4 in silica glass crucibles at 900 °C. The powder samples were characterized by X-ray diffraction, thermal analysis and FT-IR spectroscopy. Orange single crystals of the new quaternary phase were achieved by the process of crystallization with mineralizers in sealed silica glass ampoules. They were then analyzed with EDX and single-crystal X-ray analysis in which the composition Cu8In8P4O30 with the triclinic space group Pequation image (No 2) with a = 7,2429(14) Å, b = 8,8002(18) Å, c = 10,069(2) Å, α = 103,62(3)°, β = 106,31(3)°, γ = 101,55(3)° and Z = 1 was found. The three-dimensional framework consists of [InO6] octahedra and distorted [CuO6] octahedra, overcaped [InO7] prisms and [PO4] tetrahedra, also trigonal [(CuIn)O5] bipyramids and distorted [(CuIn)O6] octahedra, where copper and indium are partly exchanged against each other. Cu8In8P4O30 exhibits an incongruent melting point at 1023 °C.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Section
  5. Results and Discussion
  6. Conclusions

Searching for new cathode materials with a greater number of exchangeable electrons and a high cell voltage in combination with a suitable anodic material, the transition metal oxide phosphates have great potential. For example, the metal oxide rich copper phosphates Cu4P2O9 and Cu5P2O10 have a relatively high theoretical capacity with 466 and 496 mAh g–1. In combination with a lithium anode copper oxide phosphates achieve a discharge voltage to 3 V. In order to study the influence on the capacity and the voltage of the copper oxide phosphates, copper oxide has been partially substituted in this paper by trivalent metal oxides like In2O3. According to the literature, three quaternary phases exist in the CuO/In2O3/P2O5 system: CuIn2(P2O7)2 (monoclinic, P21/n, No 14),1,2 Cu3In2(PO4)4 (monoclinic, P21/c, No 14),35 CuInOPO4 (orthorhombic, Pnma, No 62).6 This article contributes to the investigation about further copper(II) indium(III) phosphates, especially in the metal oxide rich area of the CuO/In2O3/P2O5 system. The ternary system CuO/P2O5 is already well documented within existing literature.7,8 There are five copper(II) phosphates with the stoichiometric composition CuxP2O5+x (x = 1–5). So this work will focus on the characterization of the two systems In2O3/P2O5 and CuO/In2O3 as well as on, the determination of the completely phase relations in the CuO/In2O3/P2O5 system.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Section
  5. Results and Discussion
  6. Conclusions


The powder samples in the quaternary CuO/In2O3/P2O5 system were prepared by the calcination of the starting material and solid state reaction. Stoichiometric mixtures of CuO (pure, AppliChem. GmbH), In2O3 (99,99 %, ABCR GmbH & Co. KG) and (NH4)2HPO4 (min. 98 %, J.T. Baker Laboratory Chemicals) were heated in porcelain crucibles for 12 h at 160 °C, 12 h at 290 °C and 72 h at 900 °C.9 According to Equation (1), the reaction can be described as follows:

  • x CuO + y In2O3 + z (NH4)2HPO4 [RIGHTWARDS ARROW] “CuxIn2yPzOx+3y+2.5z” + 2z NH3 [UPWARDS ARROW] + 1.5z H2O [UPWARDS ARROW] ((1))

Additionally, a mineralization procedure was used to synthesize single crystals. The nearly pure-phase powder of the new phase and HgCl2 were placed in a sealed silica glass ampoule and heated for 160 h at 900 °C.

X-ray Powder Diffraction (XRPD)

After the preparation, the powder samples were homogenized and characterized by X-ray Diffraction with the XRD 3003 TT (General Electric) with a θ-θ-goniometer and Cu-Kα radiation (λ = 1,54 Å) in a range of 2θ = 10–80° with a step size of 0.02° s–1.

Thermal Analysis (TA)

The measurements were executed with the STA 449 C Jupiter (Netzsch) in a range from room temperature to 1400 °C with an incremental heating rate of 10 K min–1 and a sample weight between 10 and 50 mg.

Energy Dispersive X-ray Spectroscopy (EDXS)

To analyze the stoichiometric composition of the new phase, crystals from the mineralization with HgCl2 were selected and characterized by energy dispersive X-ray spectroscopy. The measurements were executed using the SEM LEO 440 (Zeiss) with an EDX detector.

Single Crystal Diffraction

A single crystal of Cu8In8P4O30 was glued to a thin glass capillary. Diffraction data for the single crystal were collected at room temperature with a diffractometer KUMA KM-4 with a CCD detector controlled by CrysAlis RED software using graphite-monochromated (monochromator Enhance, Oxford Diffraction) Mo-Kα (λ = 0.71073 Å) radiation. The data collection was performed on an orange irregular crystal with edge-lengths 0.05 × 0.01 × 0.01 mm3. The SHELXL-97 program package10 was used to determine the structure of Cu8In8P4O30. Further details of the crystal structure investigation can be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany ( on quoting the depository number CSD-425003 .

FT-IR Spectroscopy

The IR spectroscopic measurements were performed with the Digilab Scimitar FTS 2000 FT-IR Spectrometer in the wave number range of 1400–400 cm–1, using the technique of milling with an oscillating mile (Retsch MM200) in tungsten carbide milling cups for 5 min and pressing pellets of the new compound with KBr in a ratio 1:250 by weight.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Section
  5. Results and Discussion
  6. Conclusions

In Figure 1 the known ternary and quaternary phases and the investigated samples in the quasi-ternary system CuO/In2O3/P2O5 are presented. The system CuO/P2O5 is well-investigated by Gröbe.8 In order to complete the phase diagram and deriving the phase relations it was also necessary to investigate the binary systems In2O3/P2O5 and CuO/In2O3.

The Quasi-Binary System In2O3/P2O5

The three indium(III) phosphates In(PO3)3,1113 In4(P2O7)3,2,14 and InPO411,15,16 are described in the literature. The results of the phase analysis of the three indium(III) phosphates are shown in Table 1.

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Figure 1. Positions of known phases (•) and investigated samples (x) in the component-triangle of the CuO/In2O3/P2O5 system.

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Table 1. Phase analysis by XRPD of indium(III) phosphates.
Stoichiometry of the sampleX-ray phase analysisColorMelting Point /°C
In(PO3)3In(PO3)3white /light yellow1186
InPO4InPO4white> 1400

In(PO3)3 achieved pure-phase and appears white with a yellow tint. In4(P2O7)3 also obtained pure-phase. InPO4 was found in the low temperature modification (α-InPO4). The results of the phase analysis for InPO4 reflects the study by Thauern.11 The high temperature phase, β-InPO4, could be observed above a synthesis temperature of 1100 °C. In4(P2O7)3 and α-InPO4 show a white color.

Thermal analysis of In(PO3)3 showed two characteristic signals. Firstly, an endothermic signal occurs, which does not indicate any weight loss beginning at 1186 °C, secondly, another endothermic signal at 1286 °C with a weight loss of 0.5 %. It is known that CuP2O6 tends to decompose to Cu2P2O7 with rising temperature.8 In theory that means In(PO3)3 decomposes according to Equation (2):

  • 8 In(PO3)3 [RIGHTWARDS ARROW] 2 In4(P2O7)3 + 3 P4O10 [UPWARDS ARROW] ((2))

At the end of the thermal analysis of In(PO3)3 at 1400 °C the sample was molten. In4(P2O7)3 melts congruently at 1226 °C, InPO4 stays stable to 1400 °C. The phase transition from α-InPO4 to β-InPO4 was determined at 1106 °C. This result is in accordance with the preparation conditions of β-InPO4.11

The Quasi-Binary System CuO/In2O3

The copper(II) indium(III) oxides Cu2In2O51719 and CuIn2O42023 are described in the literature. Powder samples with the stoichiometric composition of the two oxides were prepared from the starting materials In2O3 and CuO at 900 °C. The phase analysis by XRPD confirmed the successful preparation of Cu2In2O5. The phase CuIn2O4 was not detectable in the product. A mixture of Cu2In2O5 and In2O3 was obtained after calcination by heating to 900 °C. The X-ray diffraction results for Cu2In2O5 correlate with the color of the sample, because an intensive green color is reported for Cu2In2O5.17 To analyze the thermal behavior of the copper indium oxide, DTA/TG-measurements were performed (Figure 2).

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Figure 2. DTA/TG-measurement of Cu2In2O5.

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In the DTA and TG curve of the diagram (Figure 2), three signals were detected. The endothermic signal at 978 °C with a low weight loss of 0.6 %, caused by an impurity of CuO in the sample of Cu2In2O5, indicates the decomposition of non reacted CuO to Cu2O.24 The next endothermic signal at 1061 °C is equivalent to the incongruent melting point of Cu2In2O5. It is known that synthesized Cu2In2O5 from CuO and In2O3 melts above 1000 °C.19 The analysis of the DTA crucible after the thermal analysis of Cu2In2O5 accorded this fact. Next to the molten mass, the strong weight loss of 2.9 % corresponds to the incongruent melting point. The following decomposition reaction describes the probable mechanism.

  • 2 Cu2In2O5 [RIGHTWARDS ARROW] 2 In2O3 + 2 Cu2O + O2 [UPWARDS ARROW] ((3))

The existence of the oxides In2O3 and Cu2O was checked by X-ray diffraction analysis of the product following the thermal analysis. Therefore it was possible to confirm the postulated reaction mechanism. The third endothermic signal at 1162 °C indicates the melting point of Cu2O. The low increase of mass (0.6 %) is caused by partial dissolving of oxygen in the Cu2O melt.

The Quasi-Ternary System CuO/In2O3/P2O5

After the calcination of the three known quaternary phases CuIn2(P2O7)2, Cu3In2(PO4)4 and CuInOPO4 appeared as powder material and were able to be confirmed by XRPD. All three phases showed the typical incongruent melting behavior similar to pure copper phosphates. The results are shown in Table 2.

Table 2. Results of phase analysis and thermal analysis of copper(II) indium(III) phosphates.
Stoichiometry of the sampleX-ray phase analysisColorMelting Point /°C
CuIn2(P2O7)2CuIn2(P2O7)2light blue1147
Cu3In2(PO4)4Cu3In2(PO4)4light green1049

Furthermore, new reflexes occurred in the X-ray powder pattern of sample compositions in the metal oxide rich region of the quasi-ternary phase diagram, which indicated a new compound. The most intensive reflexes of the unknown phase were detected at the powder composition “Cu4In4P2O15”. With the described mineralization procedure within the experiment it was possible to produce single crystals from the synthesized powder of the new phase.

Thermal Analysis of the New Quaternary Phase

Prior to this procedure it was necessary to determine a suitable growth temperature by thermal analysis. The phase “Cu4In4P2O15” shows only one endothermic effect in the thermal analytical investigation (Figure 3). The signal was able to be assigned to the incongruent melting point of the phase at 1023 °C. In the mixture of decomposition products “Cu4In4P2O15”, the adjacent quaternary phase CuInOPO4 and the oxides In2O3 and Cu2O could be identified by XRPD. The following equation describes the mechanism of decomposition.

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Figure 3. DTA/TG-measurement of “Cu4In4P2O15”.

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  • “Cu4In4P2O15[RIGHTWARDS ARROW] 2 CuInOPO4 + In2O3 + Cu2O +1/2 O2 [UPWARDS ARROW] ((4))
Energy Dispersive X-ray and Single Crystal Analysis of the New Quaternary Phase

The crystals showed an orange color and were characterized by SEM EDX. The color of the grown crystals was different than expected. But other red colored CuII containing compounds like CuAl2O425 and CuMo1–xWxO426 are known. In these structures CuII occupies only sites with distorted octahedral oxygen coordination beside tetrahedral coordination in CuAl2O4. 4 the SEM picture of one grown “Cu4In4P2O15” crystal.

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Figure 4. SEM images of a grown crystal.

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The determination of the composition by EDX on four different crystals yields to the same ratio of elements with Cu:In:P = 40:39:21 (Table 3). As a result the formula Cu4In4P2O15 was able to be derived for the new compound.

Table 3. Stoichiometric composition of the new phase by EDXS.
CrystalCu /at-%In /at-%P /at-%

The single crystal analysis yields to a formula Cu8In8P4O30(Cu8In8O14(PO4)4). The compound exhibits triclinic structure which belongs to the space group P1equation image (No 2). Detailed crystallographic data is summarized in Table 4.

Table 4. Crystallographic data for Cu8In8P4O30.
Empirical formulaCu8In8P4O30
Formula weight2030.76
Crystal systemtriclinic
Space groupP1equation image (No 2)
T /K293(2)
a 7.2429(14)
b 8.8002(18)
c 10.069(2)
Dcalc /g·cm–35.877
μ /mm–115.531
Crystal dimensions /mm0.05 × 0.01 × 0.01
Crystal colororange
Measured refls.6563
Independent refls.2622
Absorption correctionmulti-scan
No. of parameters238
Theta range/°3.34–27.49
Index ranges–9 ≤ h ≤ 9
 –11 ≤ k ≤ 11
 –13 ≤ l ≤ 13
R indices [I > 2σ(I)]: R1, wR20.0197, 0.0397
R indices (all data): R1, wR20.0263, 0.0415

The atomic coordinates and the site occupancies for the atoms of Cu8In8P4O30 are given in Table 5.

Table 5. Wyckoff sites, coordinates, site occupancies (k) and equivalent or isotropic displacement parameters (Ueq,iso).

The comparison of the calculated powder pattern from the crystallographic data shows a significant accordance to the measured powder pattern from the powder samples (Figure 5). The powder sample is almost phase-pure; only two small additional reflections at 26.0 and 35.6° were found and can be assigned to the phase CuInOPO4.

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Figure 5. Comparison of the X-ray powder pattern Cu8In8P4O30 measured vs. Cu8In8P4O30 calculated.

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Cu8In8P4O30 has a very complex structure. Five different metal oxygen polyhedra could be identified besides the [PO4] tetrahedra. Therefore, only one basic linking pattern of polyhedra between two Cu8In8P4O30 unit cells is described here (Figure 6).

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Figure 6. Crystal structure of two connected Cu8In8P4O30 unit cells (black: repeated units of edge sharing polyhedra [(CuIn)O6]-[InO6]-[(CuIn)O6], grey with black edges: [InO7]-[CuO6]/ [(CuIn)O5]-[PO4] polyhedral strings).

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[InO6] octahedra and distorted [CuO6] octahedra have been identified in the polyhedral structure of Cu8In8P4O30 which also exist in other known copper(II) indium(III) phosphates2,46 and indium(III) phosphates.12,1416 In contrast to other papers, an indium site shows a coordination number of seven respective oxygen atoms. The [InO7] polyhedron can be described as an overcaped prism. Furthermore, CuII and InIII ions are able to occupy the same crystallographic sites. Trigonal bipyramids [(CuIn)O5] and distorted octahedra [(CuIn)O6] appear in the structure of Cu8In8P4O30. This phenomenon is also described by Gruß5 in [MInO5] polyhedra of phases like M3In4(PO4)6 (M = Ni, Mg, Zn, Co). The phosphorus atom exhibits tetrahedral coordinated oxygen atoms as [PO4] polyhedron.

Along the b-direction of the Cu8In8P4O30 crystal, two parallel chains are observable that are formed by repeating units of edge sharing polyhedra [(CuIn)O6]-[InO6]-[(CuIn)O6]. The connection of the two chains is given by the following polyhedral string: [InO7]-[CuO6]/[(CuIn)O5]-[PO4], where edge shared [InO7] prisms and [PO4] tetrahedra are connected via corners with [CuO6] octahedra and [(CuIn)O5] pyramids. The [(CuIn)O6]-[InO6]-[(CuIn)O6] chains are connected at the edges with [InO7] prisms and with [PO4] tetrahedra at the corners. Figure 7 represents the basic linking pattern between two Cu8In8P4O30 unit cells for a better understanding of the complex structure.

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Figure 7. Basic linking pattern of polyhedra between two Cu8In8P4O30 unit cells.

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Looking at the bond lengths between the metal ions (copper and indium) and oxygen in the different polyhedra, shown in Table 6, it was found that they are located in an expected range when taking into account the ion radius as a basis. Typically for the disorted (stretched) octahedra are larger bond lengths by the Jahn-Teller effect in comparison with the regular octahedra.

Table 6. Bond-lengths between the metal ions and oxygen in the different polyhedra.
PolyhedraMinimum distance /ÅMaximum distance /Å
[InO7] polyhedra2.1562.396
Distorted [CuInO6] octahedra1.9422.636
Distorted [CuO6] octahedra1.9042.695
[InO6] octahedra2.0762.234
Trigonal [CuInO5] bipyramids1.9812.193
[PO4] tetrahedra1.5131.546

The analysis of the crystallographic inversion center exhibited that the inversion center is surrounded by a circle of two [PO4] tetrahedra and two [InO6] octahedra, shown in Figure 8.

FT-IR Spectrum of Cu8In8P4O30

The IR spectrum of the new compound Cu8In8P4O30 is shown in Figure 9. By applying the technique of pressing KBr pellets its quality and its visual nature are similar to other measured metal oxide phosphates. The vibrational spectrum is dominated by the fundamental vibrations of the PO43– polyanions which are split in many components due to the correlation effect induced by the coupling with Cu–O and In–O units in the structure. Following the literature, the absorption bands for the anti-symmetric and symmetric stretching vibrations of the phosphate groups in the compound Cu3In2(PO4)4 are located in the wave number range of 1180–920 cm–1.4 This is in accordance to the measured absorption in the wave number range of 1125–951 cm–1 for Cu8In8P4O30. According to the literature the absorption of Cu3In2(PO4)4,4 Cu4P2O9, and Cu5P2O10.8 respectively, below 670 cm–1 is based on the stretching vibrations of the metal–oxygen bonds and the bending vibrations of the phosphate groups. The measured IR spectrum also contained absorption signals in this range (625–410 cm–1).

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Figure 8. Description of the crystallographic inversion center.

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Figure 9. Measured IR-Spectrum Cu8In8P4O30.

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Phase Diagram of the Quasi-Ternary System CuO/In2O3/P2O5

In summary, the new compound Cu8In8P4O30, all known quaternary phases and the phase relations in the quasi-ternary system at 900 °C could be found, as shown in Figure 10.

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Figure 10. Phase diagram of CuO/In2O3/P2O5 system.

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  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Section
  5. Results and Discussion
  6. Conclusions

This report presents a systematic investigation of the phase formation and phase equilibria in the quasi-ternary system CuO/In2O3/P2O5. Powder samples were prepared using a more staged temperature regime by calcination and solid state reaction and characterized by X-ray diffraction, thermal analysis and FT-IR spectroscopy. The complete quasi-ternary system CuO/In2O3/P2O5 at 900 °C was obtained. Orange colored single crystals of the new quaternary phase were grown by crystallization with mineralizers in sealed silica glass ampoules and were analyzed by EDX and single-crystal X-ray analysis. The formula was determined to Cu8In8P4O30. The new quaternary phase crystallizes in the triclinic space group Pequation image (No 2) with a = 7,2429(14) Å, b = 8,8002(18) Å, c = 10,069(2) Å, α = 103,62(3)°, β = 106,31(3)°, γ = 101,55(3)° and Z = 1.