Oxidation state, local structure distortion, and defect structure analysis of Cu doped α-MnO2 correlated to conductivity and dielectric properties

Abstract

Cu-doped MnO2 with the composition of Mn1-xCuxO2 (x = 0–0.15) was synthesized and characterized. The synthesis was carried out by hydrothermal method at 140 °C for 5 h of reaction dwell time. The characterizations include X-Ray Diffraction (XRD), Microscopy, X-ray Absorption Spectroscopy (XAS), and Impedance complex analysis. It was revealed that all samples have nanorod morphology. Their size increases with the increasing dopant. Additionally, K ions are detected by EDX. All samples pose α-MnO2 type structures performing (2 × 2) and (1 × 1) tunnels permitting large ions incorporated and oxygen deficiency. The octahedron was distorted to elongate up to x = 0.10, then compressed for x = 0.15, inducing the Jan Teller effect. Oxidation state analysis revealed that the manganese has Mn3+ and Mn4+, while the copper is mainly attributed to Cu2+ and Cu3+ respectively. The small ionic size and highly oxidized Cu3+ substitute Mn4+, while Cu2+ substitutes Mn3+ or simultaneously with the larger K+ incorporated in the tunnel. Accordingly, the defects to exist in the sample, namely CuMn′, MnMn′, VO••, and e′. Electrical characterization at room temperature revealed that the conductivity of Cu-doped MnO2 is dominated by electrons influenced by the various oxidation state of the cations in the octahedron sites, while space charges dominate the dielectric response.

Keywords

• Manganese dioxide
• Doping
• Oxidation state
• Local distortion
• Defect structure