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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  C 2/ 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 .

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