An analysis of the crystal structure, vibrational properties, and optoelectronic behavior of NdCaFeSnO₆

The co-substitution of neodymium (Nd) and iron (Fe) into CaSnO₃ provides, for the first time, the double perovskite-like structure NdCaFeSnO₆. A comprehensive investigation of the structural, microstructural, vibrational (Raman), optical and electrical properties of the resulting NdCaFeSnO₆ compound, synthesized via a solid-state method, is reported. Elemental analysis using Energy-Dispersive X-ray spectroscopy (EDX) confirmed the successful incorporation of Nd, Fe, Ca, Sn and O. Semi-quantitative analysis results are consistent with the intended stoichiometry. X-ray diffraction (XRD) investigation revealed a monoclinic P2₁/c crystal structure, with a minor secondary phase. Rietveld refinement converges to satisfactory factors, indicating an equidistribution of Fe and Sn cations between the octahedral sites. Raman spectroscopy further characterized this symmetry, showing several bands associated with the bending and stretching modes of (Sn/Fe)O₆ octahedra. Significant narrowing of the bandgap from 4.27 eV for the pristine CaSnO₃ to 2.44 eV in NdCaFeSnO₆ supports the presence of structural defects and oxygen vacancies within the (Nd/Ca)-(Fe/Sn)-O framework. The electrical properties were elucidated with emphasis on the ionic conduction mechanism in NdCaFeSnO₆with charge carrier hopping at intermediate temperatures with an activation energy of 0.97 eV for grain boundaries and 0.31 eV for grains, based on impedance spectroscopy measurements which permit also to establish a-c and d-c conductivity contributions in this compound through Jonscher analysis. Furthermore, direct current I-V measurements revealed a Poole-Frenkel conduction mechanism at high temperatures with an activation energy of 1.55 eV. A comparison of these conduction processes was conducted to place this compound in the family of best materials for optoelectronic applications.