Comparative studies on structural and electronic transport properties of La₂ZnMnO₆, La₂NiTiO₆ and La₂NiRuO₆ double perovskites

Knowledge of electronic band structure and its role on the electronic transport as well as optical properties of semiconducting as well as insulating materials are important to characterize/evaluate their potential for practical applications. In this work, the crystallographic, morphological, electronic transport, optical and electronic band structure properties of three Lanthanum (La) based double perovskite oxides, La₂ZnMnO₆, La₂NiTiO₆ and La₂NiRuO₆ were evaluated and compared. Polycrystalline samples were synthesized by following the solid-state reaction method. Analysis of powder X-ray diffraction (XRD) data indicates monoclinic (space group: P2₁/n) crystal structure. The unit cell size of these three systems correlates with their constituent transition metal ionic sizes. Analysis of Raman spectra and Fourier transform infrared (FTIR) spectra indicates that the octahedral vibrational modes correlate with the ionic mass of the constituent transition metals for these three systems. The temperature depended resistivity data (ρ(T)) indicate semiconducting behaviour for all the three systems. Among the samples, the La₂NiRuO₆ system shows lowest ρ(T) and the La₂NiTiO₆ system shows the highest ρ(T) values. The thermal activation energy is extracted from the analysis of ρ(T) data, following the thermally activated transport model. The estimated activation energy values are 134(8) meV for La₂NiRuO₆, 203(1) meV for La₂ZnMnO₆ and 280(5) meV for La₂NiTiO₆, respectively. Also, it is observed that the normalized resistivity and activation energy of the three systems follow a direct/simple scaling trend. Additionally, diffuse reflectance spectroscopy (DRS) measurements were carried out on these systems. The DRS results further supports the observed trend in ρ(T) data among the three systems. The experimental observations is further supported by the density functional theory calculations.