Emphasis in my theoretical and computational materials chemistry research is aimed at revealing the underlying atomic and molecular mechanisms that control materials structure and response to a variety of processes so as to gain a fundamental understanding of the factors that govern chemical processes in materials and their interfaces. The desire is to provide theoretical insight into the fabrication of unique structures as a means to guide the synthesis of new materials with properties of specific interests.

Underpinning the theoretical materials chemistry toolbox are mathematical models and computational tools used to determine relationships between atomic (or molecular) level structures and their physical and chemical properties. My research methodologies employ classical and quantum statistical mechanics with an emphasis in numerical statistical mechanics based on molecular dynamics and Monte Carlo atomistic computer simulations, and computational chemistry methods to support model building efforts.

Current topics in Materials Chemistry include structure and dynamics of interfaces, irradiative modification of materials, excited states and charge transfer in lanthanide containing compounds, and the role of fluid structure and dynamics in solvating nano-particles.

Molecular Mechanisms of Hydrogen-Loaded beta-Hydroquinone Clathrate

J. Phys. Chem. B, 110, 17291, (2006) http://dx.doi.org/10.1021/jp062691c

This work performed at PNNL in collaboration with John Daschbach, Tsun-Mei Chang (U. Wisconsin - Parkside), Liem Dang, and Pete McGrail

Molecular dynamics simulations are used to investigate the molecular interactions of hydrogen-loaded beta-hydroquinone clathrate. It is found that, at lower temperatures, higher loadings are more stable, whereas at higher temperatures, lower loadings are more stable. Attractive forces between the guest and host molecules lead to a stabilized minimum-energy configuration at low temperatures. At higher temperatures, greater displacements take the system away from the shallow energy minimum, and the trend reverses. The nature of the cavity structure is nearly spherical for a loading of one, leads to preferential occupation near the hydroxyl ring crowns of the cavity with a loading of two, and at higher loadings, leads to occupation of the interstitial sites (the hydroxyl rings) between cages by a single H₂ molecule with the remaining molecules occupying the equatorial plane of the cavity. Occupation of the interstitial positions of the cavities leads to facile diffusion.

A system loaded with a single H₂ per cage behaves like a molecule solvated in a nonpolar solvent interacting weakly with the cages structure as a nearly spherical potential. However, with increased loading, the asymmetrical nature of the cage is revealed where the H₂-H₂ repulsion is sufficient to localize the H₂ positions at low temperature. At high loadings of three and four H₂ per cage, there is a preference of having one H₂ located at the interstitial position. The presence of binding sites along the equator of the cage leads to a corrugation that is observed for high loadings of three and four H₂ per cage at 20 K H₂. Finally, it is found that diffusive transport along the channel generally proceeds via a flipping, or swapping, mechanism that involves the interstitial position composed of the hydroxyl hydrogen-bonded ring. In the three and four H₂ per cage systems, a molecule in the interstitial position is always present resulting in enhanced diffusion.

Molecular dynamics simulations suggest that it should be possible to load H₂ into the beta-hydroquinone clathrate structure, possibly by loading the metastable empty structure at low temperature. The channel structure of the beta-hydroquinone clathrate results in facile diffusion along one axis at sufficiently high temperature. This channeled structure, a feature of many organic clathrates, is attractive for a material used to reversibly store H₂. It is reasonable to think that with the ability to design organic clathrates with chemical constitutes other than aromatic carbon systems may be found which provide reversible H₂ storage under mild conditions.

Characterization of exciton self-trapping in amorphous silica

J. Non-Cryst. Solids, 352, 2589 (2006) http://dx.doi.org/10.1016/j.jnoncrysol.2006.01.095

This work performed at PNNL in collaboration with Renée M. Van Ginhoven^1,2 and Hannes Jonsson^1,3

¹Department of Chemistry, University of Washington
²Currently at Sandia National Laboratory, Albuquerque, NM
³Science Institute, University of Iceland

Triplet electron–hole excitations were introduced into amorphous silica to study self-trapping (localization) and damage formation using density functional theory. Multiple self-trapped exciton (STE) states are found that can be differentiated based on the luminescence energy, the localization and distribution of the excess spin density of the triplet state, and relevant structural data, including the presence or absence of broken bonds. The trapping is shown to be affected by the relaxation response of the silica network, and by comparing results of quartz and amorphous silica systems the effects of the inherent disordered structures on exciton self-trapping are revealed. A key result is that the process of exciton trapping can lead directly to the formation of point defects, without thermal activation. The proposed mechanism includes a non-radiative decay from the excited to the ground state followed by structure relaxation to a defect configuration in the ground state.

Fig. (a) The structure of the thermally induced exciton for glass G2. The yellow spheres represent silicon atoms, and oxygen atoms are red. The green cloud indicates an isosurface of the excess spin density corresponding to the excited electron, and the dark blue cloud indicates the location of the hole. The exciton is localized at a broken bond, with an Si–O distance of 3.21 Å. The distance between the dangling oxygen atom and the nearby oxygen atom that shares the hole is 2.33 Å. (b)–(d) Structural rearrangement seen in glass G2 as a result of the action of the thermally induced exciton. (b) The initial optimized singlet state structure. (c) The thermally annealed triplet state STE structure. One Si–O bond is broken. After de-excitation back to the singlet state, atoms move to form the metastable structure seen in (d). The oxygen atoms that are bonded to different silicon atoms than in the defect-free glass are shown in black. The cut-out region shows that the new structure has a 5-fold silicon, 3-ring (on the left), and a 3-fold oxygen and edge-sharing tetrahedra (on the right). The over-coordinated silicon and oxygen atoms are 6.3 Å apart. This structure is 1.2 eV higher in energy than the defect-free structure.

• R. M. Van Ginhoven, H. Jonsson, L.R. Corrales, “Silica glass structure generation for ab initio calculations using small glass structures”, Phys. Rev. B 71, 024208 (2005).

• A. Chartier, C. Meis, J.-P. Crocombette, W.J. Weber, and L.R. Corrales, “Molecular dynamic simulation of disorder induced amorphization in pyrochlore”, Phys. Rev. Letters 94, 025505 (2005).

• R. M. Van Ginhoven, H. Jónsson, and L. R. Corrales, “Cleavage and recovery of molecular water in silica”, J. Phys. Chem. B 109, 10936 (2005).

• J. Du and L. R. Corrales, “First sharp diffraction peak in silicate glasses: Structure and scattering length dependence”, Phys. Rev. B 72, 092201 (2005).

• L. R. Corrales and J. Du, ‘Characterization of ion distributions near the surface of sodium-containing and sodium-depleted calcium aluminosilicate melts”, J. Am. Ceram. Soc. 89, 36 (2006).

• B. S. Thomas, N. A. Marks, L. R. Corrales, R. Devanathan, "Threshold displacement energies in rutile TiO2: A molecular dynamics simulation study", Nucl. Instr. and Meth. B 239 , 191-201 (2005).

• R. M. Van Ginhoven, A. Chartier, C. Meis, W. J. Weber, L. R. Corrales, “Theoretical study of helium insertion and diffusion in 3C-SiC”, J. Nucl. Mater. 348, 51 (2006).

• R. M. Van Ginhoven, H. Jónsson, and L. R. Corrales, “Characterization of exciton self-trapping in amorphous silica”, J. Non-Cryst. Solids. 352, 2589 (2006).

• J. L. Daschbach, T.-M. Chang, L. R. Corrales, L. X. Dang, P. McGrail, "Molecular Mechanisms of Hydrogen-Loaded beta-Hydroquinone Clathrate", J. Phys. Chem. B, 110, 17291 (2006).

• R. Devanathan, L. R. Corrales, W. J. Weber, A. Chartier, and C. Meis, “Molecular dynamics simulation of energetic uranium recoil damage in zircon”, Mol. Sim., In press (2006).

• J. C. Du, L. R. Corrales, "Structure, dynamics, and electronic properties of lithium disilicate melt and glass", J. Chem. Phys. 125, 114702 (2006).

• J. C. Du, L. R. Corrales, "Characterization of the structural and electronic properties of crystalline lithium silicates", J. Phys. Chem. B 110, 22346-22352 (2006).

• J. C. Du, L. R. Corrales, K. Tsemekhman, E. J. Bylaska, "Electron, hole and exciton self-trapping in germanium doped silica glass from DFT calculations with self-interaction correction", Nucl. Instr. and Meth. B 255 , 188-194 (2007).

• J. C. Du, L. R. Corrales, "Erbium implantation in silica studied by molecular dynamics simulations", Nucl. Instr. and Meth. B 255 , 177-182 (2007).

• J. C. Du, L. R. Corrales, "Understanding lanthanum aluminate glass structure by correlating molecular dynamics simulation results with neutron and X-ray scattering data", J. Non-Cryst. Solids. 353, 210-214 (2007).