Press Release

Discovery of rare-earth metal monohydride with the NaCl-type structure

May 10, 2012

Japan Atomic Energy Agency
High Energy Accelerator Research Organization, Japan
J-PARC Center, Japan
Hiroshima University, Japan

A research group of the Japan Atomic Energy Agency revealed the formation of rare-earth metal monohydride, in collaboration with groups at the High Energy Accelerator Research Organization (KEK), the Japan Proton Accelerator Research Complex (J-PARC Center), Hiroshima University, University of Tokyo, and the University of Cambridge (UK). The team utilized the high-intensity neutron beam at J-PARC and the ultra-brilliant synchrotron radiation X-rays at SPring-8, and discovered for the first time that LaH, a rare-earth metal monohydride has the NaCl-type structure.

Rare-earth metals have very high affinity for hydrogen and absorb large amounts of hydrogen atoms to form compounds (hydrides). These elements are attracting much attention as constituents of hydrogen-storage materials because hydrogen can be stored and released by the motion of hydrogen atoms in and out of the interstices in the metal lattice. Hydrogen atoms typically occupy only tetrahedral sites to form a dihydride and then occupy octahedral sites to form a trihydride; thus, all the interstices in the metal lattice are occupied (see the figure). Rare-earth metal monohydrides, in which only the octahedral sites are occupied by hydrogen, have not yet been reported.
The research group discovered that LaH₂, which is the dihydride of La, a representative rare-earth metal, decomposes into the high-hydrogen-concentration and low-hydrogen-concentration states at high pressures above 10 GPa. High pressure neutron diffraction experiments on LaD₂, in which H was replaced by D, were carried out at the Materials and Life Science Experimental Facility at J-PARC. Neutron diffraction measurement confirmed the formation of a monohydride (LaD) with low D concentration as a counterpart of an approximate trihydride (LaD₃). LaD has the NaCl-type structure, where only the octahedral sites are occupied by D atoms. First-principles calculations showed that this monohydride (LaH/LaD) is stable at high pressures.

This discovery showed that rare-earth metals comprise the only class of metals that can form monohydrides, dihydrides, and trihydrides with the face-centered cubic metal lattice. Rare-earth metals are widely used as constituents of hydrogen-storage materials because of their high affinity for hydrogen. Further detailed investigation of the metal-hydrogen bonding in the hydrides is expected to shed light on the hydrogen-metal interaction and help devise guidelines for preparing rare-earth alloys that can absorb high concentrations of hydrogen.

The SR-XRD experiments were performed under Proposal No. 2010A3703 in BL22XU and No. 2009B3610 in BL14B1 at SPring-8. The NPD experiment was performed under the S-type neutron research project (Proposal No. 2009S06) of IMSS, KEK. This work has been partially supported by the New Energy and Industrial Technology Development Organization (NEDO) under Advanced Fundamental Research on Hydrogen Storage Materials. D.Y. K. acknowledges support by the U.S. DOE BES Grant No. DE-SG0001057.

Publication Publication “Formation of NaCl-Type Monodeuteride LaD by the Disproportionation Reaction of LaD2”, Phy. Rev. Lett. Authors: A. Machida, M. Honda, T. Hattori, A. Sano-Furukawa, T. Watanuki, Y. Katayama, K. Aoki, K. Komatsu, H. Arima, H. Ohshita, K. Ikeda, K. Suzuya, T. Otomo, M. Tsubota, K. Doi, T. Ichikawa, Y. Kojima, D.Y. Kim.

Figure: Structures of the three hydrides with different hydrogen concentrations and a face-centered cubic metal lattice.
Yellow, light-blue, and dark-blue spheres indicate metal atoms, hydrogens at the octahedral sites, and hydrogens at the tetrahedral sites, respectively. (a) Monohydride with only the octahedral sites occupied; (b) dihydride with only the tetrahedral sites occupied; and (c) trihydride with both octahedral and tetrahedral sites occupied. The structure wherein only the octahedral sites are occupied is the NaCl-type structure, observed for the first time in a rare-earth metal in this work.