Condensation and Crystallization of Amorphous/Lamellar Chromium Sesquioxide

Abstract

Amorphous chromium oxide nanocondensates were fabricated by energetic pulsed laser ablation on a Cr target in vacuum for a fine particle size and a pronounced quenching effect. Analytical electron microscopy indicated the amorphous phase thus quenched has corrugated lamellar layers with bimodal interspacings of 0.259−0.266 and 0.355−0.371 nm, which are close to that of the specific lattice planes of the stable α-type structure, that is, (1̅104) and (112̅0), having the Cr-filled octahedral sites assembled as 0 and 1 periodic bond chains, respectively. Such amorphous nanocondensates were observed in situ to become more polymerized by forming (011̅2)-like layers and then fully crystallized as α-Cr2O3 for further (011̅2)-specific coalescence when irradiated by electron beam. The partially crystallized lamellae showed Raman shifts similar to that of the ambient α-Cr2O3, yet at higher frequencies due to a residual compressive stress up to ∼4 GPa. This implies a rather tight 6-coordination of Cr3+ in the corundum-like structure for the rapidly quenched amorphous phase.