La3+ doping-induced enhancement of dielectric performance in CdCu3Ti4O12 ceramics

Abstract

This study explores the structural, microstructural, electrical, and dielectric properties of Cd_1–3x/2La_xCu₃Ti₄O₁₂ ceramics (x = 0, 0.025, 0.05 mol) synthesized via the solid-state reaction method, using both experimental and theoretical approaches. X-ray diffraction analysis confirms the formation of a main structure of CaCu₃Ti₄O₁₂ in La³⁺-doped CdCu₃Ti₄O₁₂. Microstructural study reveals significant grain growth in La³⁺-doped CdCu₃Ti₄O₁₂ samples, likely due to the liquid phase sintering mechanism caused by excess Cu resulting from La³⁺ substitution, as supported by energy dispersive spectroscopy analysis. La³⁺ doping significantly enhances the dielectric permittivity, particularly at low frequencies. La³⁺-doped samples exhibit dielectric permittivity of approximately 1 × 10⁵, compared to approximately 10⁴ for undoped CdCu₃Ti₄O₁₂. Additionally, the doped samples show a low loss tangent of 0.037–0.050. This increase is attributed to space charge polarization and the formation of oxygen vacancy, supporting the internal barrier layer capacitor (IBLC) model. The x = 0.025 and 0.05 mol samples show the most stable dielectric property across −70 to 100 °C. Impedance spectroscopy confirms the IBLC structure. Impedance spectroscopy also indicates the impact of La³⁺ doping on the improved grain boundary response. X-ray photoelectron spectroscopy (XPS) indicates mixed valence states, namely Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺. These defects promote electron hopping and enhance the dielectric response. Density functional theory (DFT) calculations confirm structural stability of x = 0 and 0.05, showing that oxygen vacancies are mainly induced by external factors rather than directly by La³⁺ incorporation. Electron density analysis links the formation of Cu⁺ and Ti³⁺ to the presence of oxygen vacancy. Overall, the superior dielectric performance results from the interplay between intrinsic lattice characteristics and extrinsic defect contributions. La³⁺ doping may also lead to the formation of insulating boundary phases, contributing to reduced loss tangent. The presence of oxygen vacancy in the La³⁺-doped CdCu₃Ti₄O₁₂ structure leads to changes in the oxidation states from Cu²⁺ and Ti⁴⁺ to Cu⁺ and Ti³⁺, respectively, through a charge compensation mechanism. oxygen vacancy tends to form away from La atoms, indicating that it is not directly caused by doping but rather by external processes such as sintering. These changes result in local charge rearrangements, which enhance the electrical and dielectric properties of the material compared to undoped CdCu₃Ti₄O₁₂.