SPring-8, the large synchrotron radiation facility

The research group at Japan Atomic Energy Agency (JAEA, Director: Atsuyuki Suzuki) conducted research, in collaboration with High Energy Accelerator Research Organization (KEK, Director: Atsuto Suzuki), J-PARC Center (a research facility jointly operated by JAEA and KEK, Deputy center director: Masaji Nagamiya), Hiroshima University (president: Toshimasa Asahara), University of Tokyo (president: Junichi Hamada) and University of Cambridge (UK), on rare-earth metal^*1 hydrides, and successfully elucidated the crystal structure using the high-intensity neutron beam available at J-PARC^*2 and the synchrotron radiation X-ray available at SPring-8.^*3 The findings include the world’s first observation of a yet unknown rare-earth mono-hydride (LaH) with a rock-salt (NaCl) lattice structure.

Rare-earth metals (scarce metals) are known to have an excellent affinity to hydrogen, and often form a hydride (a metal-hydrogen compound that can contain a large amount of hydrogen). The hydrogen atom incorporated in a hydride occupies an interstice surrounded by metal atoms in one of the following two modes: a tetrahedral structure or an octahedral structure (in either case, the apex of the structure is occupied by metal atoms). Hydrogen atoms start occupying the interstices of a tetrahedral structure to form a bi-hydride, and then proceed to occupy those in an octahedral structure until total saturation (tri-hydride). Mono-hydride, where only the interstices in an octahedral structure are occupied by hydrogen atoms, is well known among transition metals,^*4 but has not been reported yet among rare-earth metals.

Prior to this study, the research group found that the di-hydride of lanthanum (LaH₂), a typical rare-earth element, existed in two distinct states, i.e. hydrogen-rich and hydrogen-poor states. This time, a series of extreme high-pressure neutron diffraction experiments were carried out at the Materials and Life Science Experimental Facility (a facility in Japan Proton Accelerator Research Complex (J-PARC)). The experiments targeted the deuterium-substituted di-hydride (LaD₂), leading to the first observation ever of the formation of two types of hydrides: a hydrogen-rich hydride that has a similar composition to tri-hydride (LaD₃), and a hydrogen-poor counterpart with a composition similar to mono-hydride (LaD). LaD takes the rock-salt (NaCl) structure, with its interstice in an octahedral structure occupied by a deuterium atom. The stable existence of mono-hydride (LaH/LaD) under high pressure was also confirmed based on a first principle calculation.^*5

The findings from this research demonstrated that only the rare-earth metals, among all metallic elements, can form three types of hydrides (mono-, di-, and tri-hydride) and all of their metallic lattices constitute a face-centered cubic structure. Owing to their excellent affinity to hydrogen, the rare-earth metals are widely used as a constituent element of hydrogen storage materials. A deeper understanding of the mechanism behind the interaction between hydrogen and metallic elements in the future, through detailed studies on hydrogen-metal binding in hydrides, holds promise with regard to helping set guidelines for developing rare-earth alloys capable of storing a large amount of hydrogen.

The research results reported here were obtained through collaboration among the researchers and institutes listed below:
JAEA: Akihiko Machida (vice principal researcher), Mitsunori Honda (special topic researcher, currently post-doctoral researcher at National Institute for Materials Science), Takanori Hattori (vice principal researcher), Asami Sano (scientist), Toru Watanuki (vice principal researcher), Yoshinori Katayama (senior scientist), Katsutoshi Aoki (special topic researcher, currently visiting researcher at Tohoku University)
J-PARC Center: Hiroshi Arima (fixed-term researcher, currently assistant professor at Tohoku University), Kentaro Suzuya (Principal researcher)
KEK: Hidetoshi Oshita (specially appointed assistant professor), Kazuki Ikeda (specially appointed associate professor), Toshiya Otomo (professor)
Hiroshima University: Masami Tsubota (specially appointed assistant professor, until March 2010), Mr. Koichi Doi, Takayuki Ichikawa (associate professor), Yoshitsugu Kojima (professor)
University of Tokyo: Kazuki Komatsu (associate professor)
University of Cambridge: Dr. Duck Young Kim (currently at Carnegie Institute for Science, USA)
A part of the research was conducted as a subject selected from the research proposals submitted to J-PARC and SPring-8, under a commission from New Energy and Industrial Technology Development Organization (NEDO), i.e. a part of the Advanced Fundamental Research on Hydrogen Storage Materials project “Experimental Elucidation of Hydrogen-material Interaction.”

The results of the research were issued in the online publication of “Physical Review Letters” (a US scientific journal) on May 8.

Publication:
"Formation of NaCl-Type Monodeuteride LaD by the Disproportionation Reaction of LaD₂"
A. Machida¹, M. Honda^2,†, T. Hattori^2,3, A. Sano-Furukawa^2,3, T. Watanuki¹, Y. Katayama¹, K. Aoki^1,‡, K. Komatsu⁴, H. Arima^3,‡, H. Ohshita⁵, K. Ikeda⁵, K. Suzuya³, T. Otomo⁵, M. Tsubota^6,§, K. Doi⁶, T. Ichikawa⁶, Y. Kojima⁶, and D. Y. Kim^7,8
1 Quantum Beam Science Directorate, Japan Atomic Energy Agency, Hyogo 679-5148, Japan
² Quantum Beam Science Directorate, Japan Atomic Energy Agency, Ibaraki 319-1195, Japan
³ J-PARC Center, Japan Atomic Energy Agency, Ibaraki 319-1195, Japan
⁴ Geochemical Research Center, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan
⁵ Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japan
⁶ Institute for Advanced Materials Research, Hiroshima University, Hiroshima 739-8530, Japan
⁷ Theory of Condensed Matter Group, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom
⁸ Geophysical Laboratory, Carnegie Institution of Washington, Washington D.C. 20015, USA
^† Present address: National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan.
^‡ Present address: Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan.
^§ Present address: High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan.
Physical Review Letters, 108, 205501 (2012), published online 14 May 2012