Formation of ultrafine and dense α-Al2O3 nanoparticles via kinetic phase change in a dynamic process

Ultrafine (5 nm) Al2O3 nanoparticles having a predominant α-type structure and with an internal compressive stress up to ca. 15 GPa were synthesized by pulsed laser ablation on Al target under a very high power density (1.8 × 1012 W/cm2) with oxygen flow in vacuum. The ultrafine α-Al2O3 was alternatively formed from the minor γ-Al2O3 nanocondensates upon electron irradiation. In such a case, the polymorphs follow a special crystallographic relationship [110]γ//[2 $$ \bar{1}$$ $$ \bar{1} $$ 0]α; ( $$ \bar{1} $$ 1 $$ \bar{1} $$ )γ//(0 $$ \bar{1} $$ 14)α with a mixed mismatch strain yet nonparallel close-packed planes indicating a reconstructive-type transformation. The formation of metastable α-Al2O3 in the dynamic processes can be rationalized by the kinetic phase change from the amorphous lamellar and/or γ-Al2O3 depending on their free energy versus cell volume curves. The dense and ultrafine-sized Al2O3 polymorphs with a rather low minimum band gap of 3.7 eV shed light on their natural occurrence in dynamic settings and abrasive as well as catalytic/optoelectronic applications.