Lattice-hydrogen interactions

A Model System for Hydrogen Interaction with a Close Packed Metal Lattice... 

Hence the statistical and thermodynamical theory of the lattice hydrogen solubility in fee fullerite with consideration for the hydrogen atoms distribution over the interstitial sites of different types has allowed us to explain and justify the formation of HX hydrofullerites with high hydrogen concentration when 0 < x < 18. It has been found that hydrogen solubility depends on the fullerite composition, its temperature, the order parameter in i = C6o, 2 = C70 fullerenes distribution over the lattice sites, the energetic constants characterizing the interaction between H- pairs at the different distances. 

Here is the momentum transferred from the neutron and x is one of the reciprocal lattice vectors of the palladium lattice. Thus, the incoherent scattering sees all the vibration modes but the coherent scattering selects one particular phonon for a particular experimental value of Q. It is now clear that both the incoherent and coherent one-phonon scattering will depend on the shape of the optical dispersion curves and hence will be influenced by hydrogen-hydrogen interactions. Indeed, one of the first observations of inelastic scattering from a hydride [10] interpreted the shape of the optical peak in terms of a frequency distribution broadened by H-H interactions. 

A model study of hydrogen interaction with the lattice of Ta has been carried out by Grena et al. through the Density Functional Theory (DFT), which allows us to describe the modifications of the Ta lattice induced by the presence of interstitial hydrogen. In particular, the DFT evaluated the elastic constants, which play an important role in the embrittlement of the hydrogenated material. The elastic constants evaluated in this work are shown in Fig. 4.16. The main result of the study was to show that the bulk modulus is unaffected by the presence of hydrogen, while the shear modulus presents a minimum (at H/M = 0.5, that is, moles of hydrogen per mole of metal) that could induce brittle behaviour. 

Simplified models for proteins are being used to predict their stmcture and the folding process. One is the lattice model where proteins are represented as self-avoiding flexible chains on lattices, and the lattice sites are occupied by the different residues (29). When only hydrophobic interactions are considered and the residues are either hydrophobic or hydrophilic, simulations have shown that, as in proteins, the stmctures with optimum energy are compact and few in number. An additional component, hydrogen bonding, has to be invoked to obtain stmctures similar to the secondary stmctures observed in nature (30). 

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