Alberto Leonardi

Common volume functions and diffraction line profiles of polyhedral domains

A general numerical algorithm is proposed for the fast computation of the common volume function (CVF) of any polyhedral object, from which the diffraction pattern of a corresponding powder can be obtained. The theoretical description of the algorithm is supported by examples ranging from simple equilibrium shapes in cubic materials (Wulff polyhedra) to more exotic non-convex shapes, such as tripods or hollow cubes. Excellent agreement is shown between patterns simulated using the CVF and the corresponding ones calculated from the atomic positions via the Debye scattering equation.

Realistic nano-polycrystalline microstructures: beyond the classical Voronoi tessellation

A modified Voronoi tessellation (MVT) is proposed for the computer simulation of realistic microstructures. Compared with standard tessellations, the present algorithm provides the desired grain size distribution in a one-step, non-evolutionary procedure. This is obtained by relaxing the constraints of Voronoi tessellation on position and orientation of the grain boundaries, with the only side effect being the formation of a limited amount of eliminable voids. As an example, it is shown how to directly obtain a distribution of grains of given variance and with a shape statistically close to the lognormal one.

On the reliability of powder diffraction Line Profile Analysis of plastically deformed nanocrystalline systems.

An iron-molybdenum alloy powder was extensively deformed by high energy milling, so to refine the bcc iron domain size to nanometer scale (~10 nm) and introduce a strong inhomogeneous strain. Both features contribute to comparable degree to the diffraction peak profile broadening, so that size and strain contributions can be easily separated by exploiting their different dependence on the diffraction angle. To assess the reliability of Line Profile Analysis, results were compared with evidence from other techniques, including scanning and transmission electron microscopy and X-ray small angle scattering. Results confirm the extent of the size broadening effect, whereas molecular dynamics simulations provide insight into the origin of the local atomic, inhomogeneous strain, pointing out the role of dislocations, domain boundaries and interactions among crystalline domains.

Directional pair distribution function for diffraction line profile analysis of atomistic models

The concept of the directional pair distribution function is proposed for an atomistic level interpretation of the line profile broadening in powder diffraction patterns of nanocrystalline materials.

Structure and morphology of shape‐controlled Pd nanocrystals

ML acknowledges support from the Italian government (Ministero dell’Istruzione, dell’Universita e della Ricerca) through the project FIRB Futuro in Ricerca RBFR10CWDA. JMF acknowledges financial support from the MINECO (Spain) project CTQ2013-44083-P and Generalitat Valenciana project PROMETEOII/2014/013.

Atomistic Model of Metal Nanocrystals with Line Defects: Contribution to Diffraction Line Profile

Molecular Dynamics (MD) was used to simulate cylindrical Pd and Ir domains with ideal dislocations parallel to the axis. Results show significant discrepancies with respect to predictions of traditional continuum mechanics. When MD atomistic models are used to generate powder diffraction patterns, strong deviations are observed from the usual paradigm of a small crystal perturbed by the strain field of lattice defects. The Krivoglaz-Wilkens model for dislocation effects of diffraction line profiles seems correct for the screw dislocation case if most parameters are known or strongly constrained. Nevertheless the practical implementation of the model, i.e., a free refinement of all microstructural parameters, leads to instability. Possible effects of the experimental practice based on Line Profile Analysis are discussed.

Interference Effects in Nanocrystalline Systems

Interference (cross-correlation) effects are present in the X-ray powder diffraction pattern of a polycrystalline aggregate. In an experimental diffraction pattern, the information is highly overlapped and can be confused with other effects. In this article, it is shown that the analysis of the patterns calculated from a cluster equilibrated via molecular dynamics simulation allows those effects to be separated. Extra intensity is observed, because of the presence of the grain boundaries contribution which is unexpectedly not that of an amorphous phase.

Diffraction line broadening from nanocrystals under large hydrostatic pressures

Atomistic copper nanocrystals were investigated via Molecular Dynamics (MD) under hydrostatic pressure to probe the relationship between applied load and structure deformation. The corresponding X-ray powder diffraction patterns were generated from the atomic coordinates. The analysis followed both the traditional Williamson-Hall approach based on pseudo-Voigt fitting and an alternative, more accurate method able to derive the integral breadths without applying a fitting. The Williamson-Hall results show discrepancies not fully associated with an issue of fitting.