Correlation between structure, chromaticity, and dielectric properties of calcium copper pyrophosphates, Ca2−xCuxP2O7

The solid-state reaction was employed to synthesize Ca2−xCuxP2O7 by varying the mole ratio between Ca and Cu. The structure and crystallography of the pyrophosphate compounds were identified and confirmed by using X-ray diffraction (XRD). The Rietveld refinement method and the extended X-ray absorption fine structure (EXAFS) least-squares fitting technique were also applied to refine the sample crystal structure. The single phases of the obtained Ca2P2O7, CaCuP2O7, and Cu2P2O7 samples and the mixing phases of the obtained Ca1.5Cu0.5P2O7 and Ca0.5Cu1.5P2O7 samples were identified, and then only a single phase of the samples was subjected to structural and dielectrical analyses. The structural results exhibit the tetragonal crystal system with the P41 space group for β-Ca2P2O7, the monoclinic crystal system with the P21/c space group for CaCuP2O7, and the C2/c space group for α-Cu2P2O7. The dielectric constant (εr) of the single metal pyrophosphates (Ca2P2O7 and Cu2P2O7) was higher than that of binary metal pyrophosphates (CaCuP2O7). The image sensor result of the Cu2P2O7 sample (x = 2.00) illustrated a yellowish-green color, while other compounds (x = 0.50−1.50) presented color tones that changed from blue-green to bluish-green. Raman and Fourier transform infrared (FTIR) spectrophotometers were employed to characterize and confirm the vibrational characteristics of the P2O74− group, which contains the O–P–O radical ([PO2]-) and the P–O–P bride ([OPO]-) and approximate M–O stretching modes. Furthermore, this work reports for the first time that the change in the crystal structure of Ca2−xCuxP2O7 (i.e., bond angle of P−O−P in P2O74− and distortion phenomena in the M−O6 octahedral site) are cause the correlation between the structure, chromaticity, and dielectric properties of calcium copper pyrophosphates, Ca2−xCuxP2O7.

www.nature.com/scientificreports/ α-Cu 2 P 2 O 7 and α-Mg 2 P 2 O 7 , exhibit a rather low sintering temperature. However, the single metal pyrophosphate groups, such as Cu 2 P 2 O 7 , Mg 2 P 2 O 7 , Zn 2 P 2 O 7 , and Co 2 P 2 O 7 , 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 M 2 P 2 O 7 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-Ca 2 P 2 O 7 is tetragonal, whereas α-Ca 2 P 2 O 7 is monoclinic 7 . Ca 2 P 2 O 7 is also an important material in the luminescence 8 and biomaterial 9 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 M 2 P 2 O 7 (M = Mg 2+ , Mn 2+ , and Zn 2+ ) compounds are difficult to sinter into dense ceramics 5 . SrZnP 2 O 7 , CaZnP 2 O 7 , α-Zn 2 P 2 O 7 , SrCuP 2 O 7 , Mn 2 P 2 O 7 , and CaCuP 2 O 7 are effective glass-free low-temperature co-fired ceramic (LTCC) materials 2,10,11 . All these metal pyrophosphates react with silver (Ag), but CaZnP 2 O 7 and SrZnP 2 O 7 do not react with copper (Cu) 5 .
Unary metal pyrophosphate, such as Mg 2 P 2 O 7, was thermally synthesized by using minerals such as dittmarite (NH 4 MgPO 4 ·H 2 O), struvite (NH 4 MgPO 4 ·6H 2 O), and newberyite (MgHPO 4 ·3H 2 O) as precursors 12 . Binary metal pyrophosphates, such as Mn 1.8 Co 0.2 P 2 O 7 , were synthesized from the thermal decomposition of manganese cobalt hydrogen phosphate trihydrate (Mn 0.9 Co 0.1 HPO 4 ·3H 2 O) 13 . Another binary metal compound, CaCuP 2 O 7 , was synthesized by using a mixture of diammonium hydrogen phosphate ((NH 4 ) 2 HPO 4 ), calcium carbonate (CaCO 3 ), and copper oxide (CuO) with the losses of carbon dioxide (CO 2 ) and ammonia (NH 3 ) gases based on the following equation (Eq. (1)) 14 : To decompose the carbonate (CO 3 2− ) and condense the phosphate (PO 4 3− ), resulting in the formation of pyrophosphate (P 2 O 7 4− ), the solid-state starting materials ((NH 4 ) 2 HPO 4 + CaCO 3 + 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, CaCuP 2 O 7 was successfully synthesized. In addition, manganese cobalt magnesium hydrogen phosphate trihydrate (Mn 0.90 Co 0.05 Mg 0.05 HPO 4 ·3H 2 O) 15  Most studies of different metal phosphate and metal pyrophosphate compounds have focused on both the syntheses and the characterizations of bulk 18,19 and nano particles 20,21 , the kinetics and thermodynamics of the reaction 22,23 , and their properties 24,25 . For example, the photoluminescence of the LiMg 0.74 Mn 0.26 PO 4 phosphor was investigated, and the results revealed that the luminescent property of this phosphor depended on its surface area 26 . 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 (Ca 2+ ) and solvent (Cu 2+ of Cu 2 P 2 O 7 ) have similar valency (Cu = Ca = 2+) and the same crystal structure (β-Cu 2 P 2 O 7 = α-Ca 2 P 2 O 7 = 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., Ca 2−x Cu x P 2 O 7 .
The dielectric properties of metal pyrophosphates occur due to two effects. They comprise the movement of M 2+ ions in the MO 6 octahedral and the shifting of O atoms in the collinear P−O−P bridge of the O 3 P−O−PO 3 or P 2 O 7 4− anion. If the collinear P−O−P bond of P 2 O 7 4− is destroyed, some distortions will also occur in the MO6 octahedra. This phenomenon will improve the dielectric properties of materials by polarization production 27 . It is well known that the highly relative permittivity of BaTiO 3 tetragonal perovskite occurs from the Ti 4+ ion off-centered in the TiO 6 octahedral.
The atomic radii of Cu 2+ and Ca 2+ are 0.73 and 1.00 Å, respectively, whereas their electronegativities are 1.90 and 1.00, respectively 28 . Doping the large cationic species, i.e., Ca 2+ , into the crystal structure of the Cu 2 P 2 O 7 host resulted in the formation of Ca 2−x Cu x P 2 O 7 solid solutions. Both distortion of the MO 6 octahedral and O shifting in the collinear P−O−P bond phenomena may occur. These phenomena may then improve the dielectric properties of Ca 2+ -doped Cu 2 P 2 O 7 compounds at low sintering temperatures. Consequently, to investigate this doubt, this research synthesized Ca 2−x Cu x P 2 O 7 (x = 0.00−2.00) by using conventional and uncomplicated methods. Then, various scientific methods were used to characterize and confirm the synthesized Ca 2−x Cu x P 2 O 7 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 Ca 2−x Cu x P 2 O 7 . 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 Ca 2−x Cu x P 2 O 7 compounds can be applied as effective optical materials. In addition, synchrotron light technology was also employed to analyze the Ca 2−x Cu x P 2 O 7 samples by using X-ray absorption spectroscopy (XAS) mode at the Cu and Ca K edges. 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 K α 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 keV 29 . 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. www.nature.com/scientificreports/

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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 Ca 2−x Cu x P 2 O 7 powders (x = 0.00−2.00) are displayed in Fig. 2. The structures of Ca 2−x Cu x P 2 O 7 were analyzed through the Rietveld refinement analytic technique 30 using the Full-Prof package 31 . 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 (Ca 2 P 2 O 7 , CaCuP 2 O 7 and Cu 2 P 2 O 7 ) was taken from parameters described well in the Calvo research 32 .
In addition, Fig. 3 shows the corresponding Rietveld refinement results of Ca 2−x Cu x P 2 O 7 when x = 0.00, 1.00, and 2.00. Figure 3 shows the calculated (Y cal ) and observed (Y obs ) diffraction patterns as well as the different values between them (Y obs −Y cal ) of the samples. The refinement plots gave the evolution of the XRD patterns in the various ratios between Ca and Cu (Ca 2−x Cu x P 2 O 7 , 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 ( The crystallographic information of the synthesized compounds is briefly described. When x = 0.00, the single metal pyrophosphate phase, β-Ca 2 P 2 O 7 , was obtained with the tetragonal crystal system, space group of P4 1 , space group number of 76, Schoenflies symbol of C 4 2,33 , and number of formula units per unit cell or Z = 8. When x = 1.00, the binary metal pyrophosphate phase, CaCuP 2 O 7 , was obtained with the monoclinic crystal system, space group of P2 1 /c, space group number of 14, Schoenflies symbol of C 2h 5 , and Z = 4. Finally, when x = 2.00, another single metal pyrophosphate phase, α-Cu 2 P 2 O 7 , was obtained with the monoclinic crystal system, space group of C2/c, space group number of 15, Schoenflies symbol of C  Table 1. 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 state 34 . Therefore, the spectra were collected at both the Ca and Cu K-edges. They could help to understand the Fourier transform evolutions 34 . The X-ray absorption edge energies (E 0 ) of the synthesized Ca 2−x Cu x P 2 O 7 compounds at the Ca and Cu K-edges are listed in Table 2.
The E 0 values of the various Cu valences (Cu 0 , Cu 1+ , and Cu 2+ ) obtained in this work are in line with the information reported by Yano and Yachandra 34 . They reported that the E 0 values increase with increasing oxidation www.nature.com/scientificreports/ 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 E 0 values (XANES spectra) shift to a higher energy 34 . According to   . 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 coordinates 36,38,39 of Ca 2−x Cu x P 2 O 7 samples (x = 0.00−2.00) are shown in Fig. 5. All Ca/Cu ratio compounds, except the composition with x = 0.00 (Ca 2 P 2 O 7 ), showed a greenish color, in which Ca 2 P 2 O 7 exhibited a colorless powder. The colors of the samples were dictated by the elongation or compression of the z ligand bonds of the Cu 2+ ion. The result of the composition with x = 2.00 (Cu 2 P 2 O 7 ) 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.
The mean static atomic dielectric constants (ε r ) of the synthesized Ca 2−x Cu x P 2 O 7 compounds were estimated using the well-known Clausius-Mossotti relation 40   www.nature.com/scientificreports/ where α D is the sum of the dielectric polarizabilities of individual ions and V m is the molar volume. The effect of porosity on the permittivity was eliminated by applying Bosman and Havinga's correction 41 as shown in Eq. (4), which can be used for some materials, i.e., dense ceramics, having porosities lower than 5%: where ε r,measured and ε r,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 (ε r ) values as a function of the composition x of the synthesized Ca 2−x Cu x P 2 O 7 (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 (Ca 2 P 2 O 7 and Cu 2 P 2 O 7 ) showed ε r values of 15.6 and 10.5, respectively, which were higher than the ε r value of binary metal pyrophosphates (i.e., CaCuP 2 O 7 , ε r = 9.8). The ε r values of the mixing phases of binary metal pyrophosphates (Ca 1.50 Cu 0.50 P 2 O 7 and 1.50 (Ca 0.50 Cu 1.50 P 2 O 7 ) 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 (Cu 2+ , Ca 2+ , and P 5+ ) and anions (O 2− ) in the crystal Ca 2−x Cu x P 2 O 7 structure. The movement of the ions in the electric field was caused (4) ε r,corrected = ε r,measured (1 + 1.5P)  www.nature.com/scientificreports/ 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 MO 6 octahedra.
The extended X-ray absorption fine structure (EXAFS) spectra of the synthesized Ca 2−x Cu x P 2 O 7 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.
Details of the EXAFS spectroscopic fitting of the Ca 2−x Cu x P 2 O 7 samples are summarized in Table 3, which shows the distortion of the CuO 6 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−O1 eq , Cu−O2 eq , Cu−O3 eq , and Cu−O4 eq, of the CuO 6 octahedral. Then, the second shell detected only one axial oxygen atom, Cu−O5 ax . The last axial oxygen atom, Cu−O6 ax, 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 structure 42,43 . 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-CaCuP 2 O 7 (x = 1.00) was less symmetric than that of another sample (Cu 2 P 2 O 7 (x = 1.00)). Second, α-CaCuP 2 O 7 (x = 1.00) exhibited four different types of atomic positions in the unit cell.
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 group 44 . After applying the Spectrum GX FTIR spectrometer, the FTIR spectra of the synthesized Ca 2−x Cu x P 2 O 7 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 literature 12,13,[15][16][17]45 Table 3. Bond length from EXAFS fitting for Ca 2−x Cu x P 2 O 7 samples; x = 1.00 and 2.00. where eq and ax subscripts are equatorial and axial (or apical) positions, respectively. CN is the coordination number, R is the radial distance, σ 2 is the mean squared displacement, and the R-factor is the fitting statistic factor. www.nature.com/scientificreports/   www.nature.com/scientificreports/ 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 (CaCuP 2 O 7 ), as described in the literature 46 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 (Ca 2 P 2 O 7 ), resulting in weak bonding, was better than the samples of x = 1.00 (CaCuP 2 O 7 ) and x = 2.00 (Cu 2 P 2 O 7 ). 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. Baur 50  where l av is the average bond length and l i 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 MO 6 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 (ε r ). These analyses showed that the polarization of Ca 2−x Cu x P 2 O 7 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 M 2+ ions in the MO 6 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.
Structural-optical relation. The distortion of the MO 6 octahedral can increase the Cu−O 6 bond lengths of Ca 2−x Cu x P 2 O 7 , resulting in an increase in the octahedral crystal field splitting energy (Δ 0 , please see Fig. 10). The Δ 0 values of the synthesized Ca 2−x Cu x P 2 O 7 samples (x = 0.50−2.00) are listed in Table 6.
As presented in Table 6, Δ 0 increased with increasing Cu 2+ fraction in the Ca 2−x Cu x P 2 O 7 compound, and when x = 2 (Cu 2 P 2 O 7 ), the highest Δ 0 value was obtained. The compounds illustrated the change in color from blue-green to bluish-green. The colorless compound, when x = 0.00 (Ca 2 P 2 O 7 ), was due to the fulfillment state in the octet rule of Ca 2+ ions in the structure, despite the distortion appearing in the CaO 6 octahedral site. The octahedral splitting diagram of Ca 2−x Cu x P 2 O 7 ; x = 0.50−2.00 is summarized and presented in Fig. 10. Total interpretations showed that the MO 6 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 (Ca 2−x Cu x P 2 O 7 ) were successfully synthesized via a solid-state reaction process. The synthesized Ca 2−x Cu x P 2 O 7 samples were systematically characterized by various scientific instruments. The structural analysis exhibits the single solid phase for the obtained Ca 2 P 2 O 7 , CaCuP 2 O 7 , and Cu 2 P 2 O 7 samples and the mixing solid phases for the obtained Ca 1.5 Cu 0.5 P 2 O 7 and Ca 0.5 Cu 1.5 P 2 O 7 samples. The tetragonal crystal system with the P4 1 space group is a crystal for β-Ca 2 P 2 O 7 , while the monoclinic crystal systems with the P2 1 /c and C2/c space groups are crystals for CaCuP 2 O 7 and α-Cu 2 P 2 O 7 , 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 MO 6 (the average bond length, octahedral volume, and distortion index) were calculated. The addition of Cu 2+ ions in the Ca 2 P 2 O 7 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 P 2