The effects of Fe-doping on MnO2: phase transitions, defect structures and its influence on electrical properties

The composition of Mn1xFexO2 (x¼0–0.15) was synthesized by a hydrothermal method at 140 C for 5 hours of reaction time. Investigations were carried out including XRD, FTIR, Raman spectroscopy, FESEM, and TEM for crystallographic phase analysis. Furthermore, XPS and XAS were used to analyze the oxidation states of Mn and dopant Fe in the octahedron sites. For electrical characterizations, an impedance analyzer was used to explore the conductivity and dielectric properties. It was discovered that the undoped MnO2 possessed an a-MnO2 structure performing (2 2) tunnel permitting K+insertion and had a nanorod morphology. The Fe ion that was doped into MnO2 caused a phase transformation from a-MnO2 to Ramsdellite R-MnO2 after x¼0.15 was reached and the tunnel dimension changed to (2 1). Furthermore, this caused increased micro-strain and oxygen vacancies. An oxidation state analysis of Mn and substituted Fe in the octahedron sites found mixed 3+ and 4+ states. Electrical characterization revealed that the conductivity of Fe-doped MnO2 is potentially electron influenced by the oxidation state of the cations in the octahedron sites, the micro-strain, the dislocation density, and the movement of K+ ions in the tunnel

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 Mn1-xCuxO2 (x ¼ 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 ¼ 0.10, then compressed for x ¼ 0.15, inducing the Jan Teller effect. Oxidation state analysis revealed that the manganese has Mn3þ and Mn4þ, while the copper is mainly attributed to Cu2þ and Cu3þ respectively. The small ionic size and highly oxidized Cu3þ substitute Mn4þ, while Cu2þ substitutes Mn3þ or simultaneously with the larger Kþ incorporated in the tunnel. Accordingly, the defects to exist in the sample, namely Cu0 Mn, Mn0 Mn, V O , and e 0 . 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 octa hedron sites, while space charges dominate the dielectric response