Probing α‑Al₂O₃: A Theoretical and Experimental Investigation of Its Optoelectronic, Thermodynamic, and Vibrational...

This work presents an integrated experimental and first-principles investigation of the structural, electronic, vibrational, optical, and thermodynamic properties of trigonal α-Al₂O₃. Experimentally, α-Al₂O₃ nanostructures were synthesized and characterized by X-ray diffraction (XRD), UV-vis spectroscopy, and Raman spectroscopy. Rietveld refinement confirmed the predominance of the trigonal corundum structure (space group R3̅c) with lattice parameters close to experimental reference values. The optical bandgap obtained from UV-vis measurements was approximately 5.6 eV, indicating the influence of nanostructuring and localized states on the optical response. Theoretical calculations were performed within density functional theory (DFT) using both LDA-CAPZ and GGA-PBE exchange-correlation functionals. The calculated electronic structure revealed dominant direct optical transitions near the Γ point with bandgap values between 6.67 and 6.68 eV. Density of states analysis showed that O-2p orbitals dominate the valence band region, while Al 3s/3p states mainly contribute to the conduction bands. Optical absorption and reflectivity spectra exhibited low anisotropy and pronounced ultraviolet absorption characteristics. Vibrational properties calculated through density functional perturbation theory (DFPT) showed good agreement with experimental Raman and infrared spectra. Phonon dispersion calculations revealed the absence of imaginary frequencies, confirming the dynamical stability of trigonal α-Al₂O₃. Thermodynamic properties, including enthalpy, entropy, free energy, and heat capacity, were evaluated up to 1000 K, indicating thermal stability and the absence of structural phase transitions within this temperature range. The results establish a consistent correlation between structural stability, electronic structure, vibrational response, and thermodynamic behavior in α-Al₂O₃ nanostructures, providing a comprehensive framework for understanding their properties and potential use in optical, catalytic, and high-temperature ceramic applications.