PROGRAMMED SYNTHESIS OF SPINEL-STRUCTURED MANGANITES EXEMPLIFIED BY Zn0.95Co0.05Mn2O4

Abstract

The study investigates the processes of phase composition formation and structural changes in spinels with the composition Zn0.95Co0.05Mn2O4, synthesized via solution combustion synthesis. The primary focus was on examining the influence of thermal treatment temperature (400–1000 °C) on the crystalline structure, phase composition, and bonding characteristics within the crystal lattice. A comprehensive analysis was conducted using powder X-ray diffraction (PXRD), infrared (IR) spectroscopy, differential thermal analysis with thermogravimetry (DTA/TG), and diffuse reflectance spectroscopy (DRS). Powder X-ray diffraction revealed that the main spinel phase Zn0.95Co0.05Mn2O4 formed even without additional calcination. Thermal treatment improved the crystalline ordering, evidenced by the narrowing of diffraction peaks and the increase in average crystallite size from 6 to 75 nm with rising calcination temperatures. Lattice parameter evaluations indicated a gradual increase in unit cell volume and c/a ratio, suggesting enhanced tetragonal distortion. IR spectroscopy confirmed the absence of organic residues in the precursor after solution combustion and identified characteristic bands for M—O (M = Zn, Co, Mn) vibrational modes in the spinel structure. Increasing calcination temperatures caused the absorption bands to shift towards higher frequencies, attributed to cation ordering and reduced lattice defects. TG/DTA analysis revealed an unusual mass gain (~23 %) up to 800 °C, associated with the oxidation of Mn²+ to Mn³+/ Mn⁴+ and oxygen incorporation into the structure. The absence of distinct exothermic or endothermic peaks indicated gradual phase changes without abrupt thermal events. Diffuse reflectance spectroscopy combined with Kubelka-Munk analysis allowed the determination of the bandgap (Eg) for samples calcined between 600 and 1000 °C. The obtained results highlight the significant impact of thermal treatment on the structural and phase characteristics of Zn0.95Co0.05Mn2O4 and provide deeper insights into the mechanisms of spinel phase formation during solution combustion and subsequent thermal treatment.