Study of phase transition behavior and high dielectric properties in YBa2_xCaxCu3O ceramics

In the current work, a full description of the strong effect of Ca substitution in Ba sites of YBa2_xCaxCu3Os ceramics prepared by the sol-gel method on structural, electrical, and dielectric properties has been presented. XRD accompanied with Rietveld refinements confirm the significant structural change from orthorhombic phase (Pmmm) for (x = 0) YBa2Cu3Os (YBCO) to tetragonal phase for (x = 2) YCa2Cu3Os with a P4/mmm spatial group. XPS was performed comparing the two extremums on the Y3d, Cu2p, and O1s, showing a shift in all peaks, confirming the change in structure. SEM study also supports the large change that will take place as Ba sites are substituted by Ca, and the correct stoichiometry of the prepared ceramics was verified with EDS analysis. Such structural changes impact directly transport properties. Temperature-dependent resistivity measurements reveal a superconducting aspect in YBa2Cu3Os whereas YCa2Cu3Os ceramics are characterized by an insulating to metallic phase transition. The electrical properties in YCa2Cu3Os phase is mainly analyzed in terms of the temperature coefficient of resistivity (the TCR effect). YCa2Cu3Os ceramics are distinguished by excellent dielectric properties, including a colossal dielectric constant (e ' r approximate to 104) accompanied by low dielectric loss over a wide range of temperatures (298K-448K) and frequencies (10-105 Hz).