First-principles study of Ti-doped sodium alanate surfaces

We have performed first-principles calculations of thick slabs of Ti-doped sodium alanate (NaAlH_4), which allows to study the system energetics as the dopant progresses from the surface to the bulk. Our calculations predict that Ti stays on the surface, substitutes for Na, and attracts a large number of H atoms to its vicinity. Molecular dynamics simulations suggest that the most likely product of the Ti-doping is the formation of H-rich TiAl_n (n>1) compounds on the surface, and hint at the mechanism by which Ti enhances the reaction kinetics of NaAlH_4.Comment: 3 pages with 3 postscript figures embedded. Uses REVTEX4 and graphicx macros. More information at http://www.ncnr.nist.gov/staff/taner/alanates

Lattice Dynamics of Solid Cubane within the Quasi-Harmonic Approximation

Solid cubane, which is composed of weakly interacting cubic molecules, exhibits many unusual and interesting properties, such as a very large thermal expansion and a first-order phase transition at T$_{p}$=394 K from an orientationally-ordered phase of R$\bar{3}$ symmetry to a {\it non-cubic} disordered phase of the same symmetry with a volume expansion of 5.4%, among the largest ever observed. We study the lattice dynamics of solid cubane within the quasi-harmonic and rigid-molecule approximation to explain some of these unusual dynamical properties. The calculated phonon density of states, dispersion curves and thermal expansion agree surprisingly well with available experimental data. We find that the amplitude of thermally excited orientational excitations (i.e. librons) increases rapidly with increasing temperature and reaches about 35$^{\rm o}$ just before the orientational phase transition. Hence, the transition is driven by large-amplitude collective motions of the cubane molecules. Similarly the amplitude of the translational excitations shows a strong temperature dependence and reaches one tenth of the lattice constant at T=440 K. This temperature is in fair agreement with the experimental melting temperature of 405 K, indicating that the Lindemann criterion works well even for this unusual molecular solid.Comment: 15 pages, 6 figures (devoted to Prof. Ciraci in honor of his sixtieth birthday

Research on Strength, Alkali-Silica Reaction and Abrasion Resistance of Concrete with Cathode Ray Tube Glass Sand

In this study, the effects on the mechanical and durability properties of concrete with cathode ray tube glass sand (CRTS) obtained by recycling the screens of cathode ray tubes (CRTs) were investigated. CRTS was used by the ratios of 5, 10, 15, and 20% in the concrete. The unit weight, workability, water absorption, compressive strength, flexural strength, ultrasonic pulse velocity, static and dynamic elastic moduli, abrasion resistance, and alkali-silica reaction (ASR) expansion tests on the concrete were examined. The use of CRTS improved specific properties of concrete according to the fraction of glass aggregate used between 0 and 20%. Plain concrete (P) and CRTS of 5% in concrete gave better results in terms of mechanical properties. Use of CRTS above 5% in concrete declined the mechanical properties but on the 90th day, CRTS concrete reduced the difference. CRTS up to 20% in concrete especially improved abrasion resistance in comparison to P without CRTS; furthermore, this addition did not increase ASR expansion to a deleterious level

Lattice Dynamics of Metal-Organic Frameworks: Neutron Inelastic Scattering and First-Principles Calculations

By combining neutron inelastic scattering (NIS) and first-principles calculations, we have investigated the lattice dynamics of metal-organic framework-5 (MOF5). The structural stability of MOF5 was evaluated by calculating the three cubic elastic constants.We find that the shear modulus, c44=1.16 GPA, is unusually small, while two other moduli are relatively large (i.e., c11=29.42 GPa and c12=12.56 GPa). We predict that MOF5 is very close to structural instability and may yield interesting phases under high pressure and strain. The phonon dispersion curves and phonon density of states were directly calculated and our simulated NIS spectrum agrees very well with our experimental data. Several interesting phonon modes are discussed, including the softest twisting modes of the organic linker

Lattice Dynamics of Solid C\u3csub\u3e60\u3c/sub\u3e

The lattice dynamics of C60 has been studied first by means of group theory and then by diagonalizing the dynamical matrix for two recently proposed intermolecular potentials. The libron and phonon energies are calculated as a function of momentum along various symmetry directions with and without phonon–libron interactions. The effects of these interactions on the density of states are also discussed. Explicit expressions for the energies of these modes at zero wave vector are given. It is found that both potential models have nearly the same phonon but a somewhat different libron spectrum. The calculated libron energies agree reasonably well with currently available experimental results