Barium substitution effects induced structural transitions, negative dielectric permittivity, and electrical conductivity behaviors in the GdCa2−xBaxCu3Oδ (0 ≤ x ≤ 2) structure

Abstract

The perovskite GdCa_2−xBa_xCu₃O_δ (0 ≤ x ≤ 2) was prepared by the conventional solid-state reaction method. This study investigates the effects of substituting calcium (Ca) with barium (Ba) in calcium-based compounds, focusing on the resultant structural, microstructural, and dielectric properties. Through systematic synthesis and characterization, we demonstrate that increasing the Ba content leads to significant alterations in the crystal structure, evidenced by X-ray diffraction (XRD) analyses. The sample acquired a transitional tetragonal/orthorhombic structure with P4/mmm and Pmmm space groups, respectively. Scanning electron microscopy (SEM) reveals microstructure changes, including grain size and morphology variations. The average grain size is found to increase with Ba doping. Scanning electron microscopy observations observe better grain connectivity with narrow grain boundaries for x = 2. Electrical conductivity and negative permittivity described the transition from dielectric to metal. As the barium content increased, the ε′ of the GdCa_2−xBa_xCu₃O_δ (6 ≤ δ ≤ 7) compound was enhanced due to interfacial polarizations at the heterogeneous interfaces, and the conductivity was improved. The ceramic with x = 0.25 exhibits higher conductivity (σ_dc ≈ 10⁻² S cm⁻¹), and a giant dielectric constant value (ε′ = 10⁴) with higher stability (30 Hz–10⁶ Hz) at room temperature. Negative permittivity has been demonstrated for conductive compounds with x = 1, 1.5, and 2. The compound's plasma oscillation caused negative permittivity to be achieved, as indicated by the Drude model. DC resistivity measurements at low temperatures confirm that samples with low Ba content (x = 0, x = 0.25, and x = 0.5) showed a metallic–insulator transition as the temperature increased. For x = 1, x = 1.5, and x = 2 the compounds become a superconductor phase. The substitution of barium shows a shift of the superconducting transition temperature (T_c) to the higher temperature region with a diminution of the resistivity.