Introduction

Currently, metal phosphate materials show interesting properties because they are used in many applications. For example, they have been applied as microwave dielectric materials, corrosion-resistant coatings, biomedical cements, chelating agents, glass ceramics, and high-quality fertilizers1,2,3. Bian et al.2 reported that metal pyrophosphates (M2P2O7, M = divalent cations) show low-loss dielectric properties as well as a relatively low sintering temperature. When the ionic radius of M in the M2P2O7 structure is higher than 0.97 Å (M = Ca2+, Sr2+, Ba2+, Pb2+, Cd2+)4, M2P2O7 compounds crystallize in the dichromate (Cr2O72−) form, in which a pair of P2O74− groups in eclipsed are the center of symmetry and bridging oxygen (O) atoms spread to each other. However, when the ionic radius of M is lower than 0.97 Å (M = Ni2+, Mg2+, Zn2+, Co2+, Cu2+, Mn2+), the M2P2O7 structure is a thortveitite type5 (scandium yttrium silicate (Sc,Y)2Si2O7 with the monoclinic crystal system, prismatic crystal class (2/m), and C2/m space group6). Based on this thortveitite structure, P2O74− groups occur in a staggered conformation. Moreover, compared to metal oxides (i.e., MO, M = divalent metals), thortveitite-type pyrophosphates, such as α-Cu2P2O7 and α-Mg2P2O7, exhibit a rather low sintering temperature. However, the single metal pyrophosphate groups, such as Cu2P2O7, Mg2P2O7, Zn2P2O7, and Co2P2O7, still show a phase transition with changing sintering temperature. Therefore, the first aim of this research is to modify the crystal structures of some metal pyrophosphate compounds to decrease the loss of the dielectric value, manipulate the relative permittivity with various temperatures, and improve the stability of the crystal structure in the high-temperature range.

The crystal structures of M2P2O7 compounds have been extensively investigated, and some metal pyrophosphates exhibit the allotropic property (a property of some compounds to exist in two or more crystal forms). For example, β-Ca2P2O7 is tetragonal, whereas α-Ca2P2O7 is monoclinic7. Ca2P2O7 is also an important material in the luminescence8 and biomaterial9 fields. The thortveitite form undergoes a reversible phase transformation below 600 °C from the α-form (occurring at low temperature) to the β-form (occurring at high temperature). However, the dichromate form undergoes irreversible transformation at temperatures above 700 °C. The thortveitite-form M2P2O7 (M = Mg2+, Mn2+, and Zn2+) compounds are difficult to sinter into dense ceramics5. SrZnP2O7, CaZnP2O7, α-Zn2P2O7, SrCuP2O7, Mn2P2O7, and CaCuP2O7 are effective glass-free low-temperature co-fired ceramic (LTCC) materials2,10,11. All these metal pyrophosphates react with silver (Ag), but CaZnP2O7 and SrZnP2O7 do not react with copper (Cu)5.

Unary metal pyrophosphate, such as Mg2P2O7, was thermally synthesized by using minerals such as dittmarite (NH4MgPO4·H2O), struvite (NH4MgPO4·6H2O), and newberyite (MgHPO4·3H2O) as precursors12. Binary metal pyrophosphates, such as Mn1.8Co0.2P2O7, were synthesized from the thermal decomposition of manganese cobalt hydrogen phosphate trihydrate (Mn0.9Co0.1HPO4·3H2O)13. Another binary metal compound, CaCuP2O7, was synthesized by using a mixture of diammonium hydrogen phosphate ((NH4)2HPO4), calcium carbonate (CaCO3), and copper oxide (CuO) with the losses of carbon dioxide (CO2) and ammonia (NH3) gases based on the following equation (Eq. (1))14:

$${2}\left( {{\text{NH}}_{{4}} } \right)_{{2}} {\text{HPO}}_{{4}} \left( {\text{s}} \right) + {\text{CaCO}}_{{3}} \left( {\text{s}} \right) + {\text{CuO}}\left( {\text{s}} \right) \to {\text{CaCuP}}_{{2}} {\text{O}}_{{7}} \left( {\text{s}} \right) + {\text{3H}}_{{2}} {\text{O}}\left( {\text{g}} \right) + {\text{4NH}}_{{3}} \left( {\text{g}} \right) + {\text{CO}}_{{2}} \left( {\text{g}} \right)$$
(1)

To decompose the carbonate (CO32−) and condense the phosphate (PO43−), resulting in the formation of pyrophosphate (P2O74−), the solid-state starting materials ((NH4)2HPO4 + CaCO3 + CuO) were homogeneously mixed first and kept at 700 °C. The obtained mixture was ground and then kept at 1060 °C for nine days. Using this thermal decomposition reaction, CaCuP2O7 was successfully synthesized. In addition, manganese cobalt magnesium hydrogen phosphate trihydrate (Mn0.90Co0.05Mg0.05HPO4·3H2O)15, manganese cobalt magnesium pyrophosphate dihydrate (Mn1.8Co0.1Mg0.1P2O7∙2H2O)16, and ammonium cobalt zinc manganese monohydrate (NH4Co0.8Zn0.1Mn0.1PO4·H2O)17 were employed as precursors to synthesize ternary metal pyrophosphates, namely, Mn1.8Co0.1Mg0.1P2O7, Mn1.8Co0.1Mg0.1P2O7, and Co1.6Zn0.2Mn0.2P2O7, respectively.

Most studies of different metal phosphate and metal pyrophosphate compounds have focused on both the syntheses and the characterizations of bulk18,19 and nano particles20,21, the kinetics and thermodynamics of the reaction22,23, and their properties24,25. For example, the photoluminescence of the LiMg0.74Mn0.26PO4 phosphor was investigated, and the results revealed that the luminescent property of this phosphor depended on its surface area26. Nevertheless, the relationship between crystal structures and dielectric properties is not widely understood. Therefore, the second aim of this work is to investigate the influence of the crystal structure on the dielectric phenomena of binary metal pyrophosphate compounds. Furthermore, substitutional solid solutions (binary metal compounds) based on the Hume-Rothery rules can be formed if the solute (Ca2+) and solvent (Cu2+ of Cu2P2O7) have similar valency (Cu = Ca = 2+) and the same crystal structure (β-Cu2P2O7 = α-Ca2P2O7 = monoclinic). This information shows a high possibility of substitutional metals between Cu and Ca ions forming a binary metal solid solution in pyrophosphate compounds, i.e., Ca2−xCuxP2O7.

The dielectric properties of metal pyrophosphates occur due to two effects. They comprise the movement of M2+ ions in the MO6 octahedral and the shifting of O atoms in the collinear P−O−P bridge of the O3P−O−PO3 or P2O74− anion. If the collinear P−O−P bond of P2O74− is destroyed, some distortions will also occur in the MO6 octahedra. This phenomenon will improve the dielectric properties of materials by polarization production27. It is well known that the highly relative permittivity of BaTiO3 tetragonal perovskite occurs from the Ti4+ ion off-centered in the TiO6 octahedral.

The atomic radii of Cu2+ and Ca2+ are 0.73 and 1.00 Å, respectively, whereas their electronegativities are 1.90 and 1.00, respectively28. Doping the large cationic species, i.e., Ca2+, into the crystal structure of the Cu2P2O7 host resulted in the formation of Ca2−xCuxP2O7 solid solutions. Both distortion of the MO6 octahedral and O shifting in the collinear P−O−P bond phenomena may occur. These phenomena may then improve the dielectric properties of Ca2+-doped Cu2P2O7 compounds at low sintering temperatures. Consequently, to investigate this doubt, this research synthesized Ca2−xCuxP2O7 (x = 0.00−2.00) by using conventional and uncomplicated methods. Then, various scientific methods were used to characterize and confirm the synthesized Ca2−xCuxP2O7 samples. Raman and Fourier transform infrared (FTIR) spectrophotometers were employed to characterize the vibrational spectra of the synthesized samples. X-ray diffraction (XRD) was used to investigate the crystal structure of the samples. The dielectric properties of the samples were also investigated by using an LCR meter, an effective technique for material measurement. The polarization phenomena in the crystal structure of the samples were studied to characterize the bond length and bond angle of Ca2−xCuxP2O7. The chromaticity property was studied by applying the image sensor with a spatially multiplexed exposure-high dynamic range (SME-HDR) imaging function. The results were then compared to the CIE (International Commission on Illumination) chromaticity diagram (standard database). Consequently, these synthesized Ca2−xCuxP2O7 compounds can be applied as effective optical materials. In addition, synchrotron light technology was also employed to analyze the Ca2−xCuxP2O7 samples by using X-ray absorption spectroscopy (XAS) mode at the Cu and Ca K edges.

Materials and methods

Preparation

Binary metal pyrophosphate samples with various Ca/Cu ratios (Ca2−xCuxP2O7, x = 0.00, 0.50, 1.00, 1.50, and 2.00) were synthesized via the solid-state method. To avoid contamination, high-purity starting materials, namely, (NH4)2HPO4 (99%), CuO (99.9%), and calcium oxide (CaO, 99.9%), were selected in this preparation process. All starting materials were weighed according to the stoichiometric ingredients and then homogenized by vibratory milling with 10 mm spherical yttria (yttrium oxide, Y2O3)-stabilized zirconia (zirconium dioxide, ZrO2) (YSZ) grinding beads in ethanol media for 4 h. The dried powders were transferred to crucibles and directly heated at 1000 °C for 24 h for Ca2−xCuxP2O7, when x = 0.00−1.50, and 800 °C for 24 h for Ca2−xCuxP2O7, when x = 2.00. After that, the calcined powders were ball-milled anew, pressed uniaxially into small pellets at a pressure of 1000 kg·cm−2 and then sintered at 950 °C for 24 h for Ca2−xCuxP2O7, when x = 0.00−1.50, and 1030 °C for 24 h for Ca2−xCuxP2O7, when x = 2.00. The observed densities of all prepared metal pyrophosphates, in theory, were investigated by Archimedes’ principle and found to be in the range of 95−98% (Fig. 1). The preparation of the target powder samples (Ca2−xCuxP2O7) was carried out according to the following reaction (Eq. (2)),

$$x{\text{CuO}}\left( {\text{s}} \right) + \left( {{2} - x} \right){\text{CaO}}\left( {\text{s}} \right) + {2}\left( {{\text{NH}}_{{4}} } \right)_{{2}} {\text{HPO}}_{{4}} \left( {\text{s}} \right) \to {\text{Ca}}_{{{2} - x}} {\text{Cu}}_{x} {\text{P}}_{{2}} {\text{O}}_{{7}} \left( {\text{s}} \right) + {\text{4NH}}_{{3}} \left( {\text{g}} \right) + {\text{3H}}_{{2}} {\text{O}}\left( {\text{g}} \right)$$
(2)

where x = 0.00−2.00.

Figure 1
figure 1

Relative theoretical densities (%) and porosities (%) of all prepared metal pyrophosphates (Ca2−xCuxP2O7; x = 0.00−2.00).

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Characterization

The room temperature FTIR spectra of the samples were recorded by using a Perkin Elmer Spectrum GX FTIR spectrometer. The measured wavenumber range was 4000−400 cm−1, whereas the selected scan number and resolution were 8 scans and 4 cm−1, respectively. A Thermo Scientific DXR Raman microscope was used to record the Raman spectra in the Raman shift of 1300−100 cm−1 using a scan number of 8 scans. A Raman spectrum was observed by irradiating each synthesized sample with an intense beam of an argon ion (Ar+) laser with a wavenumber of 20,492 cm−1 (wavelength of 488 nm). The power of the incident beam was 12.5 mW. The XRD patterns of all prepared samples were recorded by using a D8 Advance X-ray diffractometer (XRD, Bruker AXS, Karlsruhe, Germany) with Cu Kα radiation (λ = 0.1546 nm) to analyze and confirm the crystal structures of the samples. The dielectric properties were analyzed as a function of the frequency (1−1000 kHz) and temperature (room temperature to 150 °C) using an Agilent/HP 4284A precision LCR meter (an effective technique for the material measurement with a wide frequency range (20 Hz−1 MHz) and superior signal performance to test materials to the most commonly used test standards). The Sony IMX214 CMOS image sensor (CIS, 13 MP “stacked” CIS with a spatially multiplexed exposure-high dynamic range (SME-HDR) imaging function) was applied to focus the colors of the samples. The results were then compared to the CIE (International Commission on Illumination) chromaticity diagram (standard database) to estimate the trend of the absorption wavelength. X-ray absorption spectroscopy (XAS) was performed at the Beamline 8 (BL8) Station of the National Synchrotron Research Center (NSRC, Nakhon Ratchasima, Thailand). BL8 of the NSRC is routinely operated for the XAS in an intermediate photon energy range from 1.25 to 10 keV29. The double crystal Ge(220) was used for the extended X-ray absorption fine structure (EXAFS) monochromator. The XAS spectra were detected in transmission mode at the copper (Cu) and calcium (Ca) K-edge.

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Results and discussion

Structural, optical, and dielectric analyses

After applying the D8 Advance X-ray diffractometer, the resulting XRD patterns of the synthesized Ca2−xCuxP2O7 powders (x = 0.00−2.00) are displayed in Fig. 2. The structures of Ca2−xCuxP2O7 were analyzed through the Rietveld refinement analytic technique30 using the FullProf package31. A pseudo-Voigt function (a linear combination between the Lorentzian and Gaussian functions) was adequate at all times for obtaining good fits of the experimental data. The initial model for the refinement of the single phase structure (Ca2P2O7, CaCuP2O7 and Cu2P2O7) was taken from parameters described well in the Calvo research32.

Figure 2
figure 2

X-ray diffraction (XRD) patterns of the Ca2−xCuxP2O7 powders (x = 0.00−2.00) synthesized from the solid-state method of homogenized (NH4)2HPO4, CuO, and CaO.

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In addition, Fig. 3 shows the corresponding Rietveld refinement results of Ca2−xCuxP2O7 when x = 0.00, 1.00, and 2.00. Figure 3 shows the calculated (Ycal) and observed (Yobs) diffraction patterns as well as the different values between them (Yobs−Ycal) of the samples. The refinement plots gave the evolution of the XRD patterns in the various ratios between Ca and Cu (Ca2−xCuxP2O7, x = 0.00, 1.00 and 2.00). The Rietveld refinement analysis and the XRD data of powders confirmed the formation of metal pyrophosphate compounds (Ca2−xCuxP2O7).

Figure 3
figure 3

Rietveld refinement analytical results of the synthesized Ca2−xCuxP2O7 samples when x = 0.00, 1.00, and 2.00.

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The crystallographic information of the synthesized compounds is briefly described. When x = 0.00, the single metal pyrophosphate phase, β-Ca2P2O7, was obtained with the tetragonal crystal system, space group of P41, space group number of 76, Schoenflies symbol of C42,33, and number of formula units per unit cell or Z = 8. When x = 1.00, the binary metal pyrophosphate phase, CaCuP2O7, was obtained with the monoclinic crystal system, space group of P21/c, space group number of 14, Schoenflies symbol of C2h5, and Z = 4. Finally, when x = 2.00, another single metal pyrophosphate phase, α-Cu2P2O7, was obtained with the monoclinic crystal system, space group of C2/c, space group number of 15, Schoenflies symbol of C2h6, and Z = 4. For other Ca2−xCuxP2O7 samples, when x = 0.50, there were two phases between CuCaP2O7 and α-Cu2P2O7, whereas when x = 1.50, two phases between CuCaP2O7 and β-Ca2P2O7 were then observed. The P−O−P bond angles (of the O3P−O−PO3 bridge of P2O74−) and M−O6 bond lengths (M = Ca or Cu) were determined by using refinement analysis, and the obtained values are summarized in Table 1.

Table 1 Bond angles and bond lengths from the Rietveld refinement analytic technique for the synthesized Ca2−xCuxP2O7 samples; x = 0.00, 1.00, and 2.00.
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X-ray absorption near-edge structure (XANES) is very sensitive to both the change in the local geometry (especially the ligand environment of the metal) and the oxidation state34. Therefore, the spectra were collected at both the Ca and Cu K-edges. They could help to understand the Fourier transform evolutions34. The X-ray absorption edge energies (E0) of the synthesized Ca2−xCuxP2O7 compounds at the Ca and Cu K-edges are listed in Table 2.

Table 2 X-ray absorption edge energies (E0) of the synthesized Ca2−xCuxP2O7 compounds when x = 0.00−2.00.
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The E0 values of the various Cu valences (Cu0, Cu1+, and Cu2+) obtained in this work are in line with the information reported by Yano and Yachandra34. They reported that the E0 values increase with increasing oxidation state. They also described that an electron in an atom experiences the full charge of the positive nucleus. In contrast, in the case of many electrons, the electrons in an outer layer are simultaneously repelled by the negatively charged electrons and attracted to the positive nucleus. The lower the oxidation state of metals is, the less positive the overall charge of the atom. Consequently, to excite an electron from an orbital, more energy is required. In summary, when the metal has a more positive charge, the E0 values (XANES spectra) shift to a higher energy34. According to Table 2, in the Cu K-edge, the E0 values of the synthesized Ca2−xCuxP2O7 samples (x = 0.50−2.00) were similar to the E0 values of Cu2+O, indicating that Cu2+ was monoclinic. In addition, the XANES spectra of samples in the Ca K-edge showed E0 values similar to Ca2+O, indicating that there was Ca2+ in the crystal structure of the Cu2P2O7 host, resulting in the formation of Ca2−xCuxP2O7. Figure 4 presents the local environment of Ca atoms when they entered the Cu2P2O7 structure. The spectra of Ca and Cu in the CuCaP2O7 compound were different. These results demonstrated that the coordinated environments of the divalent Ca in CuCaP2O7 are significantly different35.

Figure 4
figure 4

Local environment of Ca atoms when they entered the Cu2P2O7 structure, resulting in the formation of Ca2−xCuxP2O7.

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The coordinated complexes with different properties have different colors, such as blue for Cu(NH3)4H2O)22+, red for Co(NH3)5H2O3+, and green for CoF63−. This different color phenomenon was well explained by the crystal field theory (CFT) described by El Jazouli et al. and Chen et al.36,37 The optical properties and the corresponding CIE chromatic coordinates36,38,39 of Ca2−xCuxP2O7 samples (x = 0.00−2.00) are shown in Fig. 5. All Ca/Cu ratio compounds, except the composition with x = 0.00 (Ca2P2O7), showed a greenish color, in which Ca2P2O7 exhibited a colorless powder. The colors of the samples were dictated by the elongation or compression of the z ligand bonds of the Cu2+ ion. The result of the composition with x = 2.00 (Cu2P2O7) illustrated a yellowish-green color, while the binary metal compounds (x = 0.50−1.50) presented color tones that changed from blue-green to bluish-green.

Figure 5
figure 5

Optical properties and corresponding CIE (International Commission on Illumination) chromatic coordinates of Ca2−xCuxP2O7; x = 0.00−2.00.

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The mean static atomic dielectric constants (εr) of the synthesized Ca2−xCuxP2O7 compounds were estimated using the well-known Clausius-Mossotti relation40 as the following equation (Eq. (3)):

$$\varepsilon_{r} = \left( {\frac{{3V_{m} + 8\pi \alpha_{D} }}{{3V_{m} - 4\pi \alpha_{D} }}} \right)$$
(3)

where αD is the sum of the dielectric polarizabilities of individual ions and Vm is the molar volume.

The effect of porosity on the permittivity was eliminated by applying Bosman and Havinga’s correction41 as shown in Eq. (4), which can be used for some materials, i.e., dense ceramics, having porosities lower than 5%:

$$\varepsilon_{{\text{r,corrected}}} = \varepsilon_{{\text{r,measured}}} \left( {{1} + {1}.{5}P} \right)$$
(4)

where εr,measured and εr,corrected are the measured and corrected relative permittivity, respectively, and P is the fractional porosity.

After applying the Clausius-Mossotti relation (Eq. (3)), the dielectric constant (εr) values as a function of the composition x of the synthesized Ca2−xCuxP2O7 (x = 0.00−2.00) are presented in Fig. 6, which shows the combination values between the calculated data (atomic polarization part, red bars) and measured results (atomic polarization part + ionic polarization part, red and purple bars). The single metal pyrophosphates (Ca2P2O7 and Cu2P2O7) showed εr values of 15.6 and 10.5, respectively, which were higher than the εr value of binary metal pyrophosphates (i.e., CaCuP2O7, εr = 9.8). The εr values of the mixing phases of binary metal pyrophosphates (Ca1.50Cu0.50P2O7 and 1.50 (Ca0.50Cu1.50P2O7) have not been estimated because of the unknown amount of exact phase composition. The Clausius-Mossotti equation focused on only the dielectric constant from atomic polarization (electron cloud bias in electric fields). Indeed, the samples were measured at a frequency of 1 MHz for the decreasing extrinsic factor, and the polarization caused the movement of both cations (Cu2+, Ca2+, and P5+) and anions (O2−) in the crystal Ca2−xCuxP2O7 structure. The movement of the ions in the electric field was caused by an increasing dielectric constant compared to the calculated data using the Clausius-Mossotti equation. The equation used in this study considered the dielectric constant, using the bond angle, bond length, and volume of the MO6 octahedra.

Figure 6
figure 6

Dielectric constant as a function of the composition x of Ca2−xCuxP2O7; x = 0.00−2.00.

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The extended X-ray absorption fine structure (EXAFS) spectra of the synthesized Ca2−xCuxP2O7 samples are shown in Fig. 7. The environment around Cu atoms was investigated. The primitive EXAFS model was taken from parameters obtained from the Rietveld refinement of each sample.

Figure 7
figure 7

Extended X-ray absorption fine structure (EXAFS) spectra of Cu0, Cu1+, Cu2+, Ca2+, and Ca2−xCuxP2O7; x = 0.00−1.50.

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Details of the EXAFS spectroscopic fitting of the Ca2−xCuxP2O7 samples are summarized in Table 3, which shows the distortion of the CuO6 octahedra. The spectra of x = 0.00 were undetectable because of the limitation of the instrument in beamline 8 of the National Synchrotron Research Center (Thailand). As presented in Table 3, the samples, when x = 1.00 and 2.00, showed three main shells. The first shell of the spectrum from the model consisted of four equatorial oxygen atoms, Cu−O1eq, Cu−O2eq, Cu−O3eq, and Cu−O4eq, of the CuO6 octahedral. Then, the second shell detected only one axial oxygen atom, Cu−O5ax. The last axial oxygen atom, Cu−O6ax, was observed in the third shell. The Cu atoms of Cu−O6 were also combined with the phosphorus atom Cu−P. Different radial distances (R/Å) between the Rietveld refinement and EXAFS fitting may be the cause of the measurement type of each technique. X-ray diffraction (Rietveld refinement) was used to investigate the global structure, while X-ray absorption (EXAFS fitting) was used to probe the details of the Cu/Ca local structure42,43. The fitting statistic factor (R-factor) of x = 1.00 was worse than that of x = 2.00 because of two important factors. First, the crystal structure of α-CaCuP2O7 (x = 1.00) was less symmetric than that of another sample (Cu2P2O7 (x = 1.00)). Second, α-CaCuP2O7 (x = 1.00) exhibited four different types of atomic positions in the unit cell.

Table 3 Bond length from EXAFS fitting for Ca2−xCuxP2O7 samples; x = 1.00 and 2.00.
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Vibrational spectroscopy

FTIR and Raman spectroscopies are good methods for identifying the chemical bonding of rotational, vibration, and other low-frequency modes in the phosphate group44. After applying the Spectrum GX FTIR spectrometer, the FTIR spectra of the synthesized Ca2−xCuxP2O7 samples are presented in Fig. 8, whereas the corresponding assignments are tabulated in Table 4. The FTIR spectra observed in this research are similar to the spectral results reported in the literature12,13,15,16,17,45. They successfully synthesized and investigated the vibrational spectroscopy of various single, double, and triple metal pyrophosphates, i.e., Mg2P2O7, Mn1.8Co0.2P2O7, Mn1.8Co0.1Mg0.1P2O7, Co1.6Zn0.2Mn0.2P2O7, and CoFeP2O7.

Figure 8
figure 8

Fourier transform infrared (FTIR) spectra of the synthesized Ca2−xCuxP2O7; x = 0.00−2.00.

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Table 4 Vibrational positions (wavenumber / cm−1) and vibrational assignments (modes) of the synthesized Ca2−xCuxP2O7 samples obtained from the FTIR and Raman techniques.
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The vibrational characteristics of the synthesized Ca2−xCuxP2O7 are described in detail. The strong vibrational bands at approximately 1190 and 1060 cm−1 were attributed to an asymmetric (νas PO3) vibrational mode of the PO3 unit of the pyrophosphate (O3P−O−PO34− or P2O74−) ions, whereas a vibrational band at approximately 1100 cm−1 was attributed to the symmetric stretching (νs PO3) of the PO3 unit. The asymmetric (δas PO3) and symmetric (δs PO3) bending modes are observed at the vibrational positions at approximately 580 and 540 cm−1, respectively. The asymmetric (νas P−O−P) and symmetric stretching (νs P−O−P) modes of the P−O−P bridge of the O3P−O−PO34− group were observed at vibrational positions of approximately 960 and 740 cm−1, respectively. However, in the case of the Ca2−xCuxP2O7 samples with x = 0.50 (Ca1.5Cu0.5P2O7) and x = 1.5 (Ca0.5Cu1.5P2O7), the P−O−P symmetric stretching mode appeared as two peaks in the range of 776−693 cm−1, which corresponded to the vibrational characteristics (symmetric stretching) of the P−O−P bridge. These detected peaks may be due to the mixing phases of the metal pyrophosphate compounds, i.e., Ca2P2O7 and CuCaP2O7. In addition, the rocking mode of the P−O−P deformations and the torsional and external modes were found in the 450−410 cm−1 regions.

The Raman spectroscopic technique was additionally applied to investigate and support the FTIR results, especially the vibrational spectroscopy of the metal oxide (M−O) bond as well as the lattice vibration by observation in the low frequency range (650−100 cm−1). Furthermore, the phase characteristics (α-, β-phases) of the metal pyrophosphate compounds can be observed from this spectroscopic technique. After applying the DXR Raman microscope, the Raman spectra of the samples are shown in Fig. 9, and the corresponding vibrational assignments are listed in Table 4. It was observed that the result corresponded well to the FTIR result. The Raman results showed the specific phase, which formed at high temperature in pyrophosphate with x = 1.00 (CaCuP2O7), as described in the literature46 through an undetectably weak peak at approximately 1210 cm−1. The three distinct peaks of Ca2−xCuxP2O7, where x = 0.00, 0.50, 1.50 and 2.00, which originated from the νas PO3 vibrational characteristics, were observed and found to be at approximately 1210, 1140 and 1080 cm−1. The Raman spectra clearly showed that the studied metal pyrophosphates displayed sharpness and splitting, especially in the investigated frequency region (1300−100 cm−1). The vibrational analysis of the P2O74− ion, which contained the O−P−O radical (PO2− of O2O−P−OPO34−) and the P−O−P bride (of O3P−O−PO34−), was exhibited in the Raman spectra. Moreover, M−O stretching and phase characteristics were also observed. The Raman spectra observed in this research were similar to the spectra reported by Sronsri et al.12,13,15,16,17 and Boonchom et al.45 The strong vibrational band at approximately 1100 cm−1 was attributed to the stretching of the PO3 unit of O3P−O−PO34−. The asymmetric (νasym POP) and symmetric (νsym POP) stretching types of the P−O−P bridge of O3P−O−PO34− were detected at approximately 960 and 730 cm−1, respectively. The asymmetric (δasym PO3) and symmetric (δsym PO3) vibrational bending modes of O3P−O−PO34− were observed at approximately 600 and 520 cm−1, respectively.

Figure 9
figure 9

Raman spectra of the synthesized Ca2−xCuxP2O7 compounds when x = 0.00−2.00.

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Dielectric and optical properties

Structural-dielectric relation

The bond angle and bond length were successfully investigated by using the Rietveld refinement technique, as shown in Table 1. The obtained refinement results were then used to describe the phenomena of the dielectric properties of the samples. In general, the dielectric properties of the metal pyrophosphate (M2P2O7) group occurred from two important effects, which consisted of O-atom shifting in the collinear P−O−P bridge and M2+-ion movement in the MO6 octahedral. According to previous works, due to the shifting of the O atom in the collinear P−O−P bridge, the P−O−P bond angles of Ca2P2O7 and Cu2P2O7 of 130°47 and 157°48 were reported, respectively. In this section, only three synthesized Ca2−xCuxP2O7 samples, when x = 0.00, 1.00, and 2.00, were considered. The sample, when x = 0.00 (Ca2P2O7), showed two different P−O−P bond angles. First, a bond angle of 116.52° appeared for 4 clusters per unit cell with asymmetric P−O bond lengths of 1.765 Å and 1.887 Å. Second, a P−O−P bond angle of 140.96° appeared for 4 clusters per unit cell with symmetric P−O bond lengths of 1.536 Å and 1.827 Å. The sample, when x = 2.00 (Cu2P2O7), had a P−O−P bond angle of 154.6° and 4 clusters per unit cell with a symmetric P−O bond length of 1.574 Å. Pogorzelec-Glaser et al.46 reported that at high temperature, the binary metal pyrophosphate (CuM2+P2O7) compounds crystallized in a monoclinic crystal system with the space group of C2/m, and the P−O−P angle was linear (180°). The sample, when x = 1.00 (CaCuP2O7), exhibited the space group of P21/n. The refinement result showed a P−O−P bond angle of 159.00° and 4 clusters per unit cell with asymmetric P−O bond lengths of 1.592 Å and 1.521 Å. However, the number of P−O−P clusters in each Ca2−xCuxP2O7 sample (x = 0.00, 1.00, and 2.00) was equal (4). Based on these obtained results, the P−O−P cluster number did not affect the polarization of the samples.

The single metal pyrophosphate, when x = 0.00 (Ca2P2O7), showed an outstanding dielectric constant (15.6, as shown in Fig. 6). This was a very high polarization; it therefore caused and made the narrow P−O−P bond angle. In addition, the long P−O bond length of the sample of x = 0.00 (Ca2P2O7), resulting in weak bonding, was better than the samples of x = 1.00 (CaCuP2O7) and x = 2.00 (Cu2P2O7). Additionally, the volume of the octahedral coordination was calculated using the method reported by Swanson et al.49 to present the relationship between the polarization and metal oxide bonding. In addition, the distortion index (D) was used to describe the distortion of the sample crystal structure. Baur50 described the calculation of the D value based on the bond lengths, as shown in Eq. (5).

$$D = \frac{1}{n}\sum\limits_{i = 1}^{n} {\frac{{\left| {l_{i} - l_{av} } \right|}}{{l_{av} }}}$$
(5)

where lav is the average bond length and li is the atomic distance from the central atom to the ith coordinating atom.

The refinement analysis results also showed a change in the average M−O bond lengths in the MO6 octahedral site, which caused molecular polarization. As demonstrated in Table 5, both the average bond lengths and octahedral volumes decreased with increasing x values. However, a different result was observed for the distortion index. The distortion index values increase with increasing x values, which then decreases the molecular polarization, resulting in a decrease in the dielectric constant (εr). These analyses showed that the polarization of Ca2−xCuxP2O7 occurred due to O shifting in the collinear P−O−P bridge, which is the main factor in the generation of a narrow bond angle that causes high polarization and a high dielectric constant. Moreover, the movement of M2+ ions in the MO6 octahedral was a supplementary factor, in which the longer average M−O bond length and larger octahedral volume led to the high polarization and high dielectric constant of the materials.

Table 5 Average bond length, octahedral volume, and distortion index of Ca2−xCuxP2O7 samples (x = 0.00, 1.00, and 2.00).
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Structural-optical relation

The distortion of the MO6 octahedral can increase the Cu−O6 bond lengths of Ca2−xCuxP2O7, resulting in an increase in the octahedral crystal field splitting energy (Δ0, please see Fig. 10). The Δ0 values of the synthesized Ca2−xCuxP2O7 samples (x = 0.50−2.00) are listed in Table 6.

Figure 10
figure 10

Octahedral splitting diagram of the synthesized Ca2−xCuxP2O7 samples; x = 0.50−2.00.

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Table 6 Approximate wavelength of the energy absorption.
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As presented in Table 6, Δ0 increased with increasing Cu2+ fraction in the Ca2−xCuxP2O7 compound, and when x = 2 (Cu2P2O7), the highest Δ0 value was obtained. The compounds illustrated the change in color from blue-green to bluish-green. The colorless compound, when x = 0.00 (Ca2P2O7), was due to the fulfillment state in the octet rule of Ca2+ ions in the structure, despite the distortion appearing in the CaO6 octahedral site. The octahedral splitting diagram of Ca2−xCuxP2O7; x = 0.50−2.00 is summarized and presented in Fig. 10. Total interpretations showed that the MO6 octahedral distortion affected both the color of the sample and the polarization of the octahedral unit, as reflected in the dielectric constant of the compounds.

Conclusions

Binary metal pyrophosphates (Ca2−xCuxP2O7) were successfully synthesized via a solid-state reaction process. The synthesized Ca2−xCuxP2O7 samples were systematically characterized by various scientific instruments. The structural analysis exhibits the single solid phase for the obtained Ca2P2O7, CaCuP2O7, and Cu2P2O7 samples and the mixing solid phases for the obtained Ca1.5Cu0.5P2O7 and Ca0.5Cu1.5P2O7 samples. The tetragonal crystal system with the P41 space group is a crystal for β-Ca2P2O7, while the monoclinic crystal systems with the P21/c and C2/c space groups are crystals for CaCuP2O7 and α-Cu2P2O7, respectively. The color of the samples changed from yellowish-green to bluish-green when the Cu content increased because the absorption wavelength increased and corresponded to a decrease in the z-axis expansion. Using the Rietveld refinement method, the P–O–P bond angle and P–O bond length and details of the octahedral MO6 (the average bond length, octahedral volume, and distortion index) were calculated. The addition of Cu2+ ions in the Ca2P2O7 structure resulting in distortion of the crystal structure affected the changes in the bond length and bond angle of the P–O–P groups in the P2O74− ions and the octahedral volume and average bond lengths in the octahedral MO6 site. Shifting O atoms in the collinear P–O–P bridge (a narrow bond angle) and the movement of M2+ ions in octahedral MO6 (the longer average M–O bond length and larger octahedral volume) are probably the main factors leading to the high values of polarization and dielectric constant of metal pyrophosphates. Finally, these results illustrated that the distortion of the octahedral MO6 resulted in a straightway effect on the color of the metal pyrophosphate compounds, while the change in the P–O–P bridge influenced the dielectric properties.