Correlation between structure, chromaticity, and dielectric properties of calcium copper pyrophosphates, Ca_2-xCu_xP₂O₇

The solid-state reaction was employed to synthesize Ca_2-xCu_xP₂O₇ 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 Ca_1.5Cu_0.5P₂O₇ and Ca_0.5Cu_1.5P₂O₇ 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 (ε_r) 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 P₂O₇^4- 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 Ca_2-xCu_xP₂O₇ (i.e., bond angle of P-O-P in P₂O₇^4- and distortion phenomena in the M-O₆ octahedral site) are cause the correlation between the structure, chromaticity, and dielectric properties of calcium copper pyrophosphates, Ca_2-xCu_xP₂O₇.