Ti-Zr-Ni and Ti-Hf-Ni quasicrystals and approximants as hydrogen storage alloys

X-ray and neutron powder diffraction data previously collected from the Ti-Zr-Ni quasicrystal have been analyzed. The data were modeled by an 8/5 rational approximant (RA) that was constructed using the known structure of the 1/1 RA and then refined. Ab initio total energy calculations were made for small structures with between 81 and 123 atoms, in which atomic positions were allowed to vary while minimizing the energy. The final structure is in good agreement with that obtained from the scattering data. Interstitial sites in which H is stable in the 1/1 model, and the H binding energies, were identified. An excellent fit to existing pressure-composition isotherms was obtained, but those data cannot be inverted to obtain experimental site binding energies. At 250°C the Ti-Hf-Ni alloy can be loaded with H without the formation of any detectable crystal hydride phase, which is always found in similarly loaded Ti-Zr-Ni alloys.