Correlation between structure, chromaticity, and dielectric properties of calcium copper pyrophosphates, Ca(2−x)Cu(x)P(2)O(7)

The solid-state reaction was employed to synthesize Ca(2−x)Cu(x)P(2)O(7) 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 Ca(2)P(2)O(7), CaCuP(2)O(7), and Cu(2)P(2)O(7) samples and the mixing phases of the obtained Ca(1.5)Cu(0.5)P(2)O(7) and Ca(0.5)Cu(1.5)P(2)O(7) 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 P4(1) space group for β-Ca(2)P(2)O(7), the monoclinic crystal system with the P2(1)/c space group for CaCuP(2)O(7), and the C2/c space group for α-Cu(2)P(2)O(7). The dielectric constant (ε(r)) of the single metal pyrophosphates (Ca(2)P(2)O(7) and Cu(2)P(2)O(7)) was higher than that of binary metal pyrophosphates (CaCuP(2)O(7)). The image sensor result of the Cu(2)P(2)O(7) 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 P(2)O(7)(4−) group, which contains the O–P–O radical ([PO(2)](-)) 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 Ca(2−x)Cu(x)P(2)O(7) (i.e., bond angle of P−O−P in P(2)O(7)(4−) and distortion phenomena in the M−O(6) octahedral site) are cause the correlation between the structure, chromaticity, and dielectric properties of calcium copper pyrophosphates, Ca(2−x)Cu(x)P(2)O(7).