Metastable crystalline phase in two‐dimensional metallic oxide nanoplates

Structural phase transition in crystals induced by high‐pressure or high‐temperature conditions may result in the development of unusual physical and chemical properties; these properties are often difficult to stabilize under ambient conditions. Here we adopted a simple method in which ultrathin cerium oxide nanoplates (< 1.2 nm) were synthesized to increase the surface atomic content, allowing transformation from a face‐centered cubic (fcc) phase to a body‐centered tetragonal (bct) phase. Three types of cerium oxide nanoparticles of different thicknesses, i.e., 1.2‐nm ultrathin nanoplates, 2.2‐nm nanoplates, and 5.4‐nm nanocubes, were examined using transmission electron microscopy and X‐ray diffraction. The metastable bct phase was observed only in ultrathin nanoplates. Thermodynamic energy analysis confirmed that the surface energy of the ultrathin nanoplates is the cause of the remarkable stabilization of the metastable bct phase. The mechanism of surface energy regulation can be expanded to other metallic oxides, thus providing a new means for manipulating and stabilizing novel materials under ambient conditions that otherwise would not be recovered.

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