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

Abstract

Titanium/zirconium-based quasicrystals and their related crystal approximants have been identified as potential new materials for hydrogen storage applications. To better understand the local chemistry and atomic ordering in these phases, preferential interstitial sites for hydrogen/deuterium were determined for $\ensuremath{\alpha}(\mathrm{TiCrSiO}).$ This is a bcc 1/1 crystal approximant to the icosahedral quasicrystal phase that contains a two-shell, Mackay-icosahedral cluster of atoms at each bcc site. It absorbs hydrogen or deuterium, without 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 octahedral interstitial sites are occupied with fractions of 0.14 and 0.12, respectively. Here, the hydrogen/deuterium sites are determined from a Rietveld analysis of x-ray and neutron powder diffraction data taken from samples of $\ensuremath{\alpha}(\mathrm{TiCrSiO})$ loaded with deuterium. Only the octahedral sites are occupied in the partially deuterated samples $(D/M=0.11).$ A decrease in the oxygen concentration below the stoichiometric value for $\ensuremath{\alpha}(\mathrm{TiCrSiO})$ leads to an increase in the total amount of hydrogen that can be absorbed, suggesting that these interstitial atoms are competing for the same octahedral interstitial sites.