Influence of Ca²⁺ and Mg²⁺ doping in Sr₂₋ₓCaₓNiWO₆ and Sr₂Ni₁₋ₓMgₓWO₆ double perovskite on the structural, optical, magnetic and dielectric properties / Nur Amira Farhana Mohamed Saadon

Double perovskite compounds have drawn a lot of interest and are currently the subject of intensive research due to their intriguing structural, magnetic and electrical properties. Doping with cations of various sizes in the double perovskite compounds cause changes in the crystal structure which are connected with the magnetic and electrical properties. In this study, polycrystalline samples of Sr2–xCaxNiWO6 (x=0.00, 0.02, 0.04, 0.06) and Sr2Ni1–xMgxWO6 (x=0.00, 0.02, 0.04, 0.06) were prepared through a solid state reaction method. The crystal structure, optical, magnetic and dielectric properties of the samples were studied using X-ray diffraction (XRD), scanning electron microscope (SEM), UV-Vis diffuse reflectance spectroscopy (UV-Vis), AC magnetic susceptibility (ACS) and electrochemical impedance spectroscopy (EIS) respectively. Rietveld analysis of XRD patterns for Sr2–xCaxNiWO6 series confirmed that the samples crystallized in tetragonal structures with space group I4/m. The SEM images showed that the grain sizes of the samples decreased from 2.51 μm to 1.65 μm as the dopant increased up to x=0.06. The UV-Vis analysis revealed that the band gap energy of the samples decreased from 3.17 eV to 3.11 eV up to x=0.02. The expansion of the average bond angles of <Ni–O–W> might increase the degree of overlap of Ni–O and W–O orbitals and cause the reduction in the band gap energy value. In addition, a following rising trend in the band gap energy value from 3.12 eV to 3.13 eV between x=0.04 and x=0.06 can be connected to the reduction in average particle size discovered through SEM analysis. The AC susceptibility measurement showed a magnetic transition from antiferromagnetic (AFM) to paramagnetic (PM) in the samples with the decrease of Neel temperature, TN from 33.49 K to 32.73 K. The doping of smaller size Ca2+ ions decreased the stability of the compounds’ structures by decreasing the tolerance factor from 0.9820 to 0.9810. As a result, the tilting phenomenon of NiO6 and WO6 increased which caused a slightly increased in Ni–O and W–O bond length and suppressed the antiferromagnetic interaction between Ni2+ and W6+ ions. In addition, the dielectric constant increased from ~550 to ~845 which could be attributed to a drop in crystallite size from 32.130 nm to 27.374 nm. As the crystallite size decreased, the grain boundaries increased which impeding the hopping process between the different states and grains. The Ca2+ ions concentrate at the grain boundaries and increase the dielectric constant values. For Sr2Ni1–xMgxWO6 series, the Rietveld analysis of XRD patterns confirmed that the samples crystallized in tetragonal structures with space group of I4/m. The SEM morphologies indicated that the grain size of the samples increased from 2.51 μm to 2.89 μm upon Mg2+ doping. The UV-Vis analysis revealed that the band gap energy of the samples decreased from 3.30 eV to 3.24 eV when Mg2+ ions were substituted. This illustrated that increasing ionic radii reduced the electrons’ excitation distance from valence band to conduction band. The magnetic properties of the compounds measured by AC magnetic susceptibility revealed that these compounds underwent antiferromagnetic to paramagnetic transition with the decreasing of TN values. The increase of diamagnetic, Mg2+ ions composition from 0.80% to 1.84% at the Ni2+ site which can be observed from EDX analysis has reduced the super exchange interaction between the Ni2+ and W6+ ions. Apart from that, the dielectric constant increased from ~553 to ~734 upon Mg2+ ions doping which could be attributed to the increase in grain size. Larger grain size produced greater number of dipoles and hence increased the electrical dipole moment which consequently increased the polarization effect and enhance the dielectric constant values.