Neutron diffraction determination of hydrogen atom locations in the alpha(TiCrSiO) 1/1 crystal approximant

Titanium/zirconium-based quasicrystals and their related crystal approximan ts have been identified as potential new materials for hydrogen storage app lications. To better understand the local chemistry and atomic ordering in these phases, preferential interstitial sites for hydrogen/deuterium were d etermined for alpha(TiCrSiO). This is a bcc 1/1 crystal approximant to the icosahedral quasicrystal phase that contains a two-shell, Mackay-icosahedra l cluster of atoms at each bce site. It absorbs hydrogen or deuterium, with out formation of other hydride phases, to a maximum hydrogen to metal atom ratio (H/M) of 0.26. For fully deuterated samples, both tetrahedral and oct ahedral interstitial sites are occupied with fractions of 0.14 and 0.12, re spectively. Here, the hydrogen/deuterium sites are determined from a Rietve ld analysis of x-ray and neutron powder diffraction data taken from samples of a(TiCrSiO) loaded with deuterium. Only the octahedral sites are occupie d in the partially deuterated samples (D/M = 0.11). A decrease in the oxyge n concentration below the stoichiometric value for alpha(TiCrSiO) leads to an increase in the total amount of hydrogen that can be absorbed, suggestin g that these interstitial atoms are competing for the same octahedral inter stitial sites. [S0163-1829(99)00230-1].