Synthesis, optical properties and conduction mechanism study of α- and γ-NaMnO2 materials

Abstract

Herein, NaMnO₂ cathode active materials, in both α- and γ-phases, were synthesized via a solid–solid method. Powder X-ray diffraction analysis revealed that these compounds crystallize in a monoclinic system with the C2/m space group and orthorhombic system with the Pnmm space group for the α- and γ-phases, respectively. Raman spectroscopy showed the presence of the MnO₆ octahedral group in the α-phase, and the MnO₄ tetrahedral group in the γ-phase. The direct band gap (E_g) was calculated to 4.96 eV and 5.14 eV for the α- and γ-phases, respectively, confirming that α-NaMnO₂ has better electronic conductivity than γ-NaMnO₂. The Urbach energy values, E_u, show that the octahedral α-phase is more disordered than the tetrahedral γ-phase in the NaMnO₂ material. The electrical data analysis of the impedance spectra and the imaginary part of the complex modulus showed the presence of two types of relaxations, corresponding to grain and grain boundary effects in both compounds. The thermal evolution of the relaxation time and DC conductivity followed the Arrhenius law with a change in the activation energy at around T_α = 363 K for the α-phase and T_γ = 373 K for the γ-phase. In addition, the frequency behaviour of the AC conductivity, σ_ac, of these two samples was analyzed using the universal Jonscher law. Therefore, the variation in s_g for both compounds with temperature showed a change in the conduction mechanism at around T_α and T_γ for the α- and γ-phases, respectively.