Gilles A. de Wijs

Report on the sixth blind test of organic crystal structure prediction methods

The results of the sixth blind test of organic crystal structure prediction methods are presented and discussed, highlighting progress for salts, hydrates and bulky flexible molecules, as well as on-going challenges.

Anisotropy of the mobility of pentacene from frustration

The bandstructure of pentacene is calculated using first-principles density functional theory. A large anisotropy of the hole and electron effective masses within the molecular planes is found. The band dispersion of the HOMO and the LUMO is analyzed with the help of a tight-binding (TB) fit. The anisotropy is shown to be intimately related to the herringbone structure.

DFT study of planar boron sheets: a new template for hydrogen storage

We study the hydrogen storage properties of planar boron sheets and compare them to those of graphene. The binding of molecular hydrogen to the boron sheet (0.05 eV) is stronger than that to graphene. We find that dispersion of alkali metal (AM = Li, Na, and K) atoms onto the boron sheet markedly increases hydrogen binding energies and storage capacities. The unique structure of the boron sheet presents a template for creating a stable lattice of strongly bonded metal atoms with a large nearest neighbor distance. In contrast, AM atoms dispersed on graphene tend to cluster to form a bulk metal. In particular, the boron−Li system is found to be a good candidate for hydrogen storage purposes. In the fully loaded case, this compound can contain up to 10.7 wt % molecular hydrogen with an average binding energy of 0.15 eV/H2.

Tunable hydrogen storage in magnesium-transition metal compounds: first-principles calculations

Magnesium dihydride (MgH2) stores 7.7 wt % hydrogen but it suffers from a high thermodynamic stability and slow (de)hydrogenation kinetics. Alloying Mg with lightweight transition metals (TM) (=Sc,Ti,V,Cr) aims at improving the thermodynamic and kinetic properties. We study the structure and stability of MgxTM1−xH2 compounds, x=[0–1], by first-principles calculations at the level of density functional theory. We find that the experimentally observed sharp decrease in hydrogenation rates for x0.8 correlates with a phase transition of MgxTM1−xH2 from a fluorite to a rutile phase. The stability of these compounds decreases along the series Sc, Ti, V, and Cr. Varying the TM and the composition x, the formation enthalpy of MgxTM1−xH2 can be tuned over the substantial range of 0–2 eV/f.u. Assuming however that the alloy MgxTM1−x does not decompose upon dehydrogenation, the enthalpy associated with reversible hydrogenation of compounds with a high magnesium content (x=0.75) is close to that of pure Mg.

Hydrogen Storage by Polylithiated Molecules and Nanostructures

We study polylithiated molecules as building blocks for hydrogen storage materials, using first-principles calculations. CLi4 and OLi2 bind 12 and 10 hydrogen molecules, respectively, with an average binding energy of 0.10 and 0.13 eV, leading to gravimetric densities of 37.8 and 40.3 wt % of H2. Bonding between Li and C or O is strongly polar and H2 molecules attach to the partially charged Li atoms without dissociating, which is favorable for (de)hydrogenation kinetics. CLin and OLim molecules can be chemically bonded to graphene sheets to hinder the aggregation of such molecules. In particular B- or Be-doped graphene strongly bind the molecules without seriously affecting the hydrogen binding energy. This still leads to a hydrogen storage capacity in the range of 5−8.5 wt % of H2.

A solid-state NMR and DFT study of compositional modulations in AlxGa1-xAs

We have conducted (75)As and (69)Ga Nuclear Magnetic Resonance (NMR) experiments to investigate order/disorder in Al(x)Ga(1-x)As lift-off films with x∼ 0.297 and 0.489. We were able to identify all possible As(Al(n)Ga(4-n)) sites with n = 0-4 coordinations in (75)As NMR spectra using spin-echo experiments at 18.8 Tesla. This was achieved by employing high rf field strengths using a small solenoid coil and an NMR probe specifically designed for this purpose. Spectral deconvolution, using an evolutionary algorithm, complies with the absence of long-range order if a CuAu based order parameter is imposed. An unconstrained fit shows a deviation of the statistics imposed by this type of ordering. The occupational disorder in the Ga and Al positions is reflected in a distribution of the Electric Field Gradients (EFGs) experienced at the different arsenic sites. We established that this can be modelled by summing the effects of the first coordination sphere and a Czjzek type distribution resulting from the compositional variation in the Al/Ga sub-lattice in the higher coordination spheres. (69)Ga 3QMAS and nutation data exclude the presence of highly symmetric sites and also show a distribution in EFG. The experimentally obtained quadrupolar interactions are in good agreement with calculations based on Density Functional Theory (DFT). Using additivity of EFG tensors arising from distant charge perturbations, we could use DFT to model the EFG distributions of the n = 0-4 sites, reproducing the Czjzek and extended Czjzek distributions that were found experimentally. On the basis of these calculations we conclude that the (75)As quadrupolar interaction is sensitive to compositional modulations up to the 7th coordination shell in these systems.

First principles modelling of magnesium titanium hydrides

Mixing Mg with Ti leads to a hydride Mg(x)Ti((1 - x))H(2) with markedly improved (de)hydrogenation properties for x ≤ 0.8, as compared to MgH(2). Optically thin films of Mg(x)Ti((1 - x))H(2) have a black appearance, which is remarkable for a hydride material. In this paper we study the structure and stability of Mg(x)Ti((1 - x))H(2), x = 0-1 by first-principles calculations at the level of density functional theory. We give evidence for a fluorite to rutile phase transition at a critical composition x(c) = 0.8-0.9, which correlates with the experimentally observed sharp decrease in (de)hydrogenation rates at this composition. The densities of states of Mg(x)Ti((1 - x))H(2) have a peak at the Fermi level, composed of Ti d states. Disorder in the positions of the Ti atoms easily destroys the metallic plasma, however, which suppresses the optical reflection. Interband transitions result in a featureless optical absorption over a large energy range, causing the black appearance of Mg(x)Ti((1 - x))H(2).

Phonons and electron-phonon coupling in graphene-h-BN heterostructures

First principle calculations of the phonons of graphene-h-BN heterostructures are presented and compared to those of the constituents. We show that AA and AB’ stacking are not only energetically less favoured than AB but also dynamically unstable. We have identied low energy at phonon branches of h-BN character with out of plane displacement and evaluated their coupling to electrons in graphene.

Symmetry, Dynamics, and Defects in Methylammonium Lead Halide Perovskites

In order to better understand the structure and dynamics of methylammonium lead halide perovskites, we performed NMR, NQR, and DFT studies of CH3NH3PbI3 in the tetragonal and cubic phase. Our results indicate that the space group of the tetragonal phase is the nonpolar I4/mcm. The highly dynamic methylammonium moiety shows no indication of the occurrence of additional orientations of the C-N bond close to the c-axis at temperatures approaching the cubic phase. Crystal quality effects are shown to influence the 14N NMR and 127I NQR spectra, and the effects of high-temperature annealing on defects can be observed. A strong increase in T2 relaxation time of the 207Pb NMR signal on cooling is found, and is an indication of slow motions in the PbI6 octahedra at room temperature. These results aid in the understanding of the structure of methylammonium lead halides and enable further studies of defects in these materials.

Theoretical study of the stable radicals galvinoxyl, azagalvinoxyl and Wurster's blue perchlorate in the solid state

Calculations on crystalline organic radicals were performed to establish the ground states of these materials. These calculations show that the radicals may interact, depending on their orientation in the crystal structure. For galvinxoyl, a second structure is proposed which is similar to that of azagalvinoxyl, in which the radicals form pairs. This structure accounts for the anomalous magnetic properties of galvinoxyl at low temperatures.