RESUMEN
Nanometric multilayers composed of immiscible Ag and Ni metals were investigated by means of molecular dynamics simulations. The semi-coherent interface between Ag and Ni was examined at low temperatures by analyzing in-plane strain and defect formation. The relaxation of the interface under annealing conditions was also considered. With increasing temperature, a greater number of atomic planes participated in the interface, resulting in enhanced mobility of Ag and Ni atoms, as well as partial dissolution of Ni within the amorphous Ag. To mimic polycrystalline layers with staggered grains, a system with a triple junction between a silver single layer and two grains of nickel was examined. At high temperatures (900 K and 1000 K), the study demonstrated grain boundary grooving. The respective roles of Ni and Ag mobilities in the first steps of grooving dynamics were established. At 1100 K, a temperature close but still below the melting point of Ag, the Ag layer underwent a transition to an amorphous/premelt state, with Ni grains rearranging themselves in contact with the amorphous layer.
RESUMEN
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.
RESUMEN
The stacking parameters, lattice constants, and bond lengths of solvent-free dilithium squarate (Li(2)C(4)O(4)) crystals were investigated using density functional theory with and without dispersion corrections. The shortcoming of the GGA (PBE) calculation with respect to the dispersive forces appears in the form of an overestimation of the unit cell volume up to 5.8%. The original Grimme method for dispersion corrections has been tested together with modified versions of this scheme by changing the damping function. One of the modified dispersion-corrected DFT schemes, related to a rescaling of van der Waals radii, provides significant improvements for the description of intermolecular interactions in Li(2)C(4)O(4) crystals: the predicted unit cell volume lies then within 0.9% from experimental data. We applied this optimised approach to the screening of hypothetical framework structures for the delithiated (LiC(4)O(4)) and lithiated (Li(3)C(4)O(4)) phases, i.e. oxidized and reduced squarate forms. Their relative energies have been analysed in terms of dispersion and electrostatic contributions. The most stable phases among the hypothetical models for a given lithiation rate were selected in order to calculate the corresponding average voltages (either upon lithiation or delithiation of Li(2)C(4)O(4)). A first step towards energy partitioning in view of interpretating crystal phases relative stability in link with (de)-intercalation processes has been performed through the explicit evaluation of electrostatic components of lattice energy from atomic charges gained with the Atoms in Molecules (AIM) method.