Structure and stability of high pressure synthesized Mg–TM hydrides (TM = Ti, Zr, Hf, V, Nb and Ta) as possible new hydrogen rich hydrides for hydrogen storage

David Moser; Daniel James Bull; Toyoto Sato; Dag Noréus; Daisuke Kyoi; Tetsuo Sakai; Naoyuki Kitamura; Hitoshi Yusa; Takashi Taniguchi; Willem Peter Kalisvaart; Peter Notten

doi:10.1039/B911263D

Structure and stability of high pressure synthesized Mg–TM hydrides (TM = Ti, Zr, Hf, V, Nb and Ta) as possible new hydrogen rich hydrides for hydrogen storage

David Moser,*^a Daniel James Bull,^a Toyoto Sato,†^b Dag Noréus,^b Daisuke Kyoi,^c Tetsuo Sakai,^c Naoyuki Kitamura,^c Hitoshi Yusa,^d Takashi Taniguchi,^d Willem Peter Kalisvaart^e and Peter Notten^e

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* Corresponding authors

^a Institute for Materials Research, University of Salford, Salford, Greater Manchester, UK
E-mail: d.moser@salford.ac.uk

^b Department of Structural Chemistry, Stockholm University, Stockholm, Sweden

^c Research Institute for Ubiquitous Energy Devices, Advanced Industrial Science and Technology, Osaka 563-8577, Japan

^d Advanced Materials Laboratory, National Institute for Material Science (NIMS), 1-1 Namiki, Tsukuba, Japan

^e Eindhoven University of Technology, Eindhoven, Netherlands

Abstract

A series of hydrogen rich Mg₆_–7TMH_14–16 (TM = Ti, Zr, Hf, V, Nb and Ta) hydrides have been synthesized at 600 °C in a high pressure anvil cell above 4 GPa. All have structures based on a fluorite type metal atom subcell lattice with a ≈ 4.8 Å. The TM atom arrangements are, however, more ordered and can best be described by a superstructure where the 4.8 Å FCC unit cell axis is doubled. The full metal atom structure corresponds to the Ca₇Ge type structure. This superstructure was also observed from electron diffraction patterns. The hydrogen atoms were found from powder X-ray diffraction using synchrotron radiation to be located in the two possible tetrahedral sites. One coordinates three Mg atoms and one TM atom and another coordinates four Mg atoms. These types of new hydrogen rich hydrides based on immiscible metals were initially considered as metastable but have been observed to be reversible if not fully dehydrogenated. In this work, DFT calculations suggest a mechanism whereby this can be explained: with H more strongly bonded to the TM, it is in principle possible to stepwise dehydrogenate the hydride. The remaining hydrogen in the tetrahedral site coordinating the TM would then act to prevent the metals from separating, thus making the system partially reversible.

Journal of Materials Chemistry

Structure and stability of high pressure synthesized Mg–TM hydrides (TM = Ti, Zr, Hf, V, Nb and Ta) as possible new hydrogen rich hydrides for hydrogen storage

Abstract

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Structure and stability of high pressure synthesized Mg–TM hydrides (TM = Ti, Zr, Hf, V, Nb and Ta) as possible new hydrogen rich hydrides for hydrogen storage

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