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The size-dependent phase stability of gamma-Al2O3 was studied by large-scalemolecular dynamics simulations over a wide temperature range from 300 to 900 K.For the gamma-Al2O3 crystal, a bulk transformation to alpha-Al2O3 by anFCC-to-HCP transition of the O sublattice is still kinetically hindered at 900K. However, local distortions of the FCC O-sublattice by the formation ofquasi-octahedral Al local coordination spheres become thermally activated, asdriven by the partial covalency of the Al-O bond. On the contrary, sphericalgamma-Al2O3 NPs (with sizes of 6 and 10 nm) undergo a crystalline-to-amorphoustransformation at 900 K, which starts at the reconstructed surface andpropagates into the core through collective displacements of anions andcations, resulting in the formation of 7- and 8-fold local coordination spheresof Al. In parallel, the reconstructed Al-enriched surface is separated from thestoichiometric core by a diffuse Al-depleted transition region. Thiscompositional heterogeneity creates a disbalance of charges inside the NP,which induces a net attractive Coulombic force that is strong enough to reversethe initial stress state in the NP core from compressive to tensile. Thesefindings disclose the delicate interplay between lattice distortions, stresses,and space-charge regions in oxide nanosystems. A fundamental explanation forthe reported expansion of metal-oxide NPs with decreasing size is provided,which has significant implications for e.g. heterogeneous catalysis, NPsintering, and additive manufacturing of NP-reinforced metal matrix composites.

Atomistic simulations of the crystalline-to-amorphous transformation of  gamma-Al2O3 nanoparticles: delicate interplay between lattice distortions,  stresses, and space charges