1 research outputs found
Atomistic Simulations of the Crystalline-to-Amorphous Transformation of γ‑Al<sub>2</sub>O<sub>3</sub> Nanoparticles: Delicate Interplay between Lattice Distortions, Stresses, and Space Charges
The size-dependent phase stability
of γ-Al2O3 was studied by large-scale
molecular dynamics simulations
over a wide temperature range from 300 to 900 K. For the γ-Al2O3 crystal, a bulk transformation to α-Al2O3 by an FCC-to-HCP transition of the O sublattice
is still kinetically hindered at 900 K. However, local distortions
of the FCC O-sublattice by the formation of quasi-octahedral Al local
coordination spheres become thermally activated, as driven by the
partial covalency of the Al–O bond. On the contrary, spherical
γ-Al2O3 nanoparticles (NPs) (with sizes
of 6 and 10 nm) undergo a crystalline-to-amorphous transformation
at 900 K, which starts at the reconstructed surface and propagates
into the core through collective displacements of anions and cations,
resulting in the formation of 7- and 8-fold local coordination spheres
of Al. In parallel, the reconstructed Al-enriched surface is separated
from the stoichiometric core by a diffuse Al-depleted transition region.
This compositional heterogeneity creates an imbalance of charges inside
the NP, which induces a net attractive Coulombic force that is strong
enough to reverse the initial stress state in the NP core from compressive
to tensile. These findings disclose the delicate interplay between
lattice distortions, stresses, and space-charge regions in oxide nanosystems.
A fundamental explanation for the reported expansion of metal-oxide
NPs with decreasing size is provided, which has significant implications
for, e.g., heterogeneous catalysis, NP sintering, and additive manufacturing
of NP-reinforced metal matrix composites