A detailed analysis of the formation energies for alkali, earth-alkali, and transition-metal hydrides is presented. The hydriding energies are computed for various crystal structures using density functional theory. The early transition metals are found to have a strong tendency for hydride formation which decreases as one goes to the right in the transition-metal series. A detailed analysis of the changes in band structure and electron density upon hydride formation has allowed us to understand the hydriding energy on the basis of three contributions. The first is the energy to convert the crystal structure of the metal to the structure formed by the metal ions in the hydride ͑fcc in most cases͒. In particular, for metals with a strong bcc preference such as V and Cr, this significantly lowers the driving force for hydride formation. A second contribution, which for some materials is dominant, is the loss of cohesive energy when the metal structure is expanded to form the hydride. This expansion lowers the cohesive energy of the metal and is a significant impediment to form stable hydrides for the middle to late transition metals, as they have high cohesive energies. The final contribution to the hydride formation energy is the chemical bonding between the hydrogen and metal in which it is inserted. This is the only contribution that is negative and hence favorable to hydride formation.
A comprehensive understanding of nanotube materials requires the ability to link different carbon nanotube models, which were developed to work at different length scales. Here we describe the mapping of a molecular dynamics (MD) model for single-wall carbon nanotubes onto a wormlike chain. This mapping employs a mode analysis of the bending fluctuations of the nanotube, similar to those used in experiments [1]. The essence of this mapping is to find an appropriate bending stiffness for the wormlike cha in in order to represent the nanotube on a coarsened scale. We find that this mapping will only work well, if the wavelength probing the nanotube stiffness is sufficiently large. For single-wall (9,9) armchair nanotubes vibration modes with node distances of 3 nm underestimate the long wavelength limit of the bending constant by about 50%. This mismatch tends to increase for tubes with larger radii.
A detailed analysis of the formation energies of transition metal hydrides is presented. The hydriding energies are computed for various crystal structures using Density Functional Theory. The process of hydride formation is broken down into three consecutive, hypothetical reactions in order to analyse the different energy contributions, and explain the observed trends. We find that the stability of the host metal is very significant in determining the formation energy, thereby providing a more fundamental justification for Miedema's “law of inverse stability” [1] (the more stable the metal, the less stable the hydride). The conversion of the host metal to the structure formed by the metal ions in the hydride (fcc in most cases) is only significant for metals with a strong bcc preference such as V and Cr - this lowers the driving force for hydride formation. The final contribution is the chemical bonding between the hydrogen and the metal. This is the only contribution that is negative, and hence favourable to hydride formation. We find that it is dominated by the position of the Fermi level in the host metal.
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