Effect of calcination temperature on structural, optical, and morphological properties of RAlO₃ (R = La, Sm) perovskite oxides

Abstract

RAlO3 (R = La, Sm) have attracted the research community due to their interesting optoelectronic properties and viable applications. The solution combustion method allows for a faster process and lower calcination temperature than the traditional solid-state method and is an economical alternative to wet chemical synthesis for producing RAlO3. This work explores the thermal, structural, morphological, optical, and electrical properties of RAlO3 (R = La, Sm) synthesised by the solution combustion method using urea as fuel. Thermogravimetric analysis – differential thermal analysis (TGA-DTA) showed the crystallization temperatures of lanthanum aluminate (LaAlO3) and samarium aluminate (SmAlO3) at 864 and 887 °C, respectively. The x-ray diffraction (XRD) and Rietveld analysis revealed the structure of LaAlO3 as rhombohedral (R-3c) and SmAlO3 as orthorhombic (Pbnm). The LaAlO3 and SmAlO3 samples calcinated at 800 °C showed crystallite size (D) of 19.26 nm and 19.06 nm, respectively. The field emission scanning electron microscopy (FE-SEM) images show a highly porous and large sheet-like morphology with voids and cracks. High resolution transmission electron microscopy (HR-TEM) images and the selective area electron diffraction (SAED) pattern show crystalline nature and the indexed planes agreed with the XRD results. The LaAlO3 and SmAlO3 samples had specific surface areas of 16.374 and 12.953 m2/g, respectively. The TGA-DTA results were affirmed by Fourier transform infrared spectroscopy (FT-IR) results which showed only the presence of metal-oxide bonds for materials annealed at and above 800 °C. These results were further validated by the electron dispersive x-ray spectroscopy (EDX) and the x-ray photoelectron spectroscopy (XPS) showing no additional peaks. The band gap of 5.08 and 4.82 eV were calculated from ultraviolet-visible spectroscopy (UV-Vis) for LaAlO3 and SmAlO3, respectively. The results imply that the solution combustion technique using urea as fuel is a viable route for synthesising RAlO3.