Synthesis of Ultrafine Aluminum-Rich Corundum Nanocrystals

Nanocrystalline Cr_2xAl_2(1-x)O₃ solid solutions offer a unique combination of chemical and thermal stabilities with optoelectronic and mechanical properties of interest for applications in catalysis, medicine, and structural materials. However, there exists an asymmetric miscibility gap in the range 0.02 < x < 0.7 at temperatures below 1000 °C. Since sintering is extremely fast at high temperatures, synthesis of Al-rich corundum nanocrystals less than 50 nm in diameter has been elusive. In this work, we present a bottom-up synthesis of homogeneous α-Cr_2xAl_2(1-x)O₃ nanocrystals with ultimate grain sizes as low as 20 nm. X-ray diffraction, electron microscopy, and energy-dispersive X-ray spectroscopy were used to demonstrate complete mixing of Cr and Al in the particles. Temperatures as low as 850 °C resulted in complete homogenization of α-Cr_2xAl_2(1-x)O₃ nanocrystals. The ultrafine powders exhibited photoluminescence with a sharp emission line at 695 nm when excited by a 532 nm laser, characteristic of Cr³⁺ substitutions in the α-Al₂O₃ lattice. Additionally, the characteristic decay time of this emission was increased from 132.7 μs to 278.5 μs when the Cr concentration was diluted from Cr_0.4Al_1.6O₃ to Cr_0.02Al_1.98O₃