Influence of the A-site cation on structural, morphological, magnetic, optical and transport properties of R2CoMnO6(R = Nd, Y, Ho and Er) double perovskite oxides

Abstract

The A-site substituted double perovskite oxides (DPOs) have in the recent decades gained much attention due to their various interesting properties and possible applications. For this reason, R2CoMnO6 (R = Nd, Y, Ho, and Er) DPOs have been synthesized using the auto-combustion sol-gel method. X-ray diffraction analysis suggests the crystal structure, i.e., monoclinic for all under-investigated DPOs at room temperature (RT), consisting of space group P2 1 /n which is confirmed by Rietveld refinement. Crystallite size is calculated using Scherrer and Williamson-Hall methods, and it was found that the crystallite size decreased with decreasing ionic radii from 55–29 nm on average. The micrographs were obtained by using a field-emission scanning electron microscope (FE-SEM), which reveals an almost homogeneous distribution of grains throughout the surface of all DPOs. All observed infrared and Raman active phonon modes have been assigned to vibrations of atoms and groups of atoms consistent with the prediction of group theory. The intermediate band gap values (1.34–1.63 eV) determined by the Tauc relations suggest the semiconducting nature of these DPOs which makes them promising materials for photovoltaic applications. Dc electrical resistivity and dielectric measurements also suggest semiconducting behaviour above ambient temperature, which is attributed to a small polaron hopping conduction mechanism. The Maxwell-Wagner type interfacial polarization has described the nature of the frequency-dependent dielectric constants in these DPOs. The dielectric (ac) study also suggests the small polaron hopping conduction mechanism in all samples, which supports our findings in resistivity measurements.