Dr Stefano Leoni
Nucleation in the Solid State
The detailed investigation of structural reconstruction, in pressure or temperature induced phase transitions, is a major challenge in modern material sciences. The combined use of structural modeling approaches and of different advanced numerical tools allows for a detailed understanding of phase nucleation and growth in the solid state. Therein, intermediate reconstruction steps can be elucidated in detail.
Figure 1: Nucleation and growth of a crystalline pattern from a pristine one: CsCl to Rocksalt phase transition in RbCl. Many nuclei are forming, which interact to build the final material. The result of the calculations is a detailed map of solid state reactivity, containing shape of the nuclei, local atomic rearrangements, reaction rates, and which can provide a realistic visualization of the final material (lower left corner), including domains and grain boundaries
Chemical Bond Reconstruction
Bond reconstruction during phase transitions. In phosphorus (black-P to α-As type) reconstruction takes the form of interfacial Peierls-like chains (Fig. 2a), in germanium the hR8 to cI16 reconstruction takes place as SN2 reaction (Fig. 2b), in carbon (graphite to sp3 polymorphs) as a complex pattern of odd-membered ring formation. In every system considered so far, a clear indication of bond nucleation is apparent. In carbon, different mechanisms are responsible for the formation of distinct sp3 polymorphs (Fig. 3c-d). Each mechanism in turn is initiated by different nucleation pattern, promptly suggesting ways of controlling/influencing the formation of either compound.
Figure 2: Covalent network reconstruction of the elements a) phosphorus, b) germanium, c-d) carbon. a) A17-A7 reconstruction of layered phosphorus: formation of an intermediate Peierls-like (red) chain, triggered by single bond formation, interconnecting the layers. b) Reconstruction of the hR8 network into the cI16 network in germanium: formation of two distinct sets of intertwined Ge-Ge linkages (red and blue). The blue ones contain the reactive, SN2-type bond inversion processes; red ones are on the contrary non-reactive. c-d) Formation of sp3 carbons from graphite, under the effect of cold compression. Distinct nucleation pattern lead to different final topologies. In c), rapid bond growth (vertically in figure) anticipates the formation of five-membered rings, followed by seven-membered ring closure. In d) odd rings are formed more sparsely and locally into a pattern that leads to a distinct sp3 carbon. Clearly, nucleation history affects the accessibility of a particular product, as we learn form simulations.
Novel Carbon Materials
Carbon remains a most versatile material. It has the potential of providing clean and very effective solutions in different areas like hydrogen storage, capacitors, optical and electronic devices. The polymorphism of carbon has been the object of many discoveries over the years. Novel forms, ranging from extended (graphene) to finite (nanotubes and fullerenes) have appeared, with outstanding properties. The polymorphism of carbon can be the source of even more interesting properties. Our recent results are along three lines: a) systematic computational approach to carbon polymorphisms, b) novel superhard and transparent sp3 carbon materials, c) carbon nanotubes assemblies with superior hydrogen storage properties. We start from sp2 carbons, graphite and nanotubes to achieve different forms of sp3 carbons on the one hand; on the other hand we address the question of packing nanotubes (CNTs) in space, with a dramatic boost on their properties as hydrogen adsorbers.
Figure 3: a) Four novel carbons were found from metaD runs. All phases are stackings of corrugated graphene layers interconnected by alternating sequences of odd or even rings. Ring topologies influence hardness and optical propertie. b) CNTs matrices display outstanding hydrogen storage properties. At room temperature they can adsorb up to 5.5 wt.% at 100 bar, which closely approaches the Department of Energy (DOE) target value of 6 wt.%. The process is dominated at its early stage by nucleation at strong absorptions sites, followed by hydrogen filling in space.
Metallorganic framework compounds (MOFs)
MOFs are porous molecular scaffoldings able to accommodate guest molecules, and hydrogen in particular. We have developed and expertise in constructing not-yet-synthesized MOFs, and in assessing their capacities as molecular reservoirs. We have started a systematic investigation of promising hydrogen storage candidate materials to meet this challenge. We focus on open framework zeolitic materials like MOFs, COFs and recently ZIFs. MOFs and ZIFs are mechanically versatile and easy to synthesize. On combining topological enumeration, molecular dynamics and structure optimization we are proposing a catalogue with useful hydrogen uptakes. Upon chemical substitution we are able to enhance their specificity for hydrogen, for instance by introducing polarizing centers in the framework (LiB(imid)4, BIFs). In general it is possible to explore a large number of structure candidates, introduce chemical substitution, and evaluate as storage properties in a systematic and efficient way.
Figure 4: The estimation of hydrogen uptake in MOFs, COFs and ZIFs materials can be reliably calculated using Grand canonical Monte Carlo molecular simulations, which indicate LiB(imid)4 structures based on fau, rho, and gme nets as promising candidates for hydrogen storage applications. Their total hydrogen uptake at 77 K amounts to 7.8, 6.9 and 6.9 wt.%, respectively. Note that hydrogen uptake of the fau-based LiB(imid)4 is comparable to that of MOF-177 (~10.0 wt. %), which is a reference material among experimentally characterized compounds.
Battery materials: Mass and charge transport
Two are the principal computational problems in this area: a correct evaluation of mass diffusion/transport, and a reliable ground-state electronic structure (also for resistivity calculations). Our recent work on Li diffusion in an important battery material LiFePO4 represents a step forward in assessing properties and calculating relevant parameters, like diffusion constants, and represents a viable method for accelerating diffusion, which is a typical bottleneck for MD simulations. This extends the possibilities of molecular dynamics to approaching systems where particle diffusion is the dominating physical/chemical event. It allows collecting a number of relevant trajectories on the one had: on the other, details of local oxidation/reduction steps at metal centers can be addressed.
Figure 5: Different aspects of the computation of material properties for cathode materials. a) Diffusion paths of Li within FePO4. In the presence of antisite disorder, besides the main paths (vertical in figure), the  direction (horizontal) is also activated. b) The calculation of the correct ground state by means of DFT+ Hubbard U methods is the starting point for the evaluation of resistivity profiles for charged and discharged FePO4/LiFePO4. The presence of peaks in the dielectric function is related to well-defined oxidations states, Fe2+/Fe3+ for instance.