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

Cu-doped MnO₂ with the composition of Mn_1-xCu_xO₂ (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 Cu_Mn^′, Mn_Mn^′, V_O^••, 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.