Correlation between structure, chromaticity, and dielectric properties of calcium copper pyrophosphates, Ca2\u2212xCuxP2O7

Abstract

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.