Structures of the icosahedral clusters in Ni-Nb-Zr-H glassy alloys determined by first-principles molecular dynamics calculation and XAFS measurements

To elucidate the hydrogen localization sites in the atomic clusters of Ni-Nb-Zr-H glassy alloys exhibiting room-temperature Coulomb oscillation, we investigated the effect of hydrogen on the electronic state of the elements forming the glassy alloys, as well as the coordinating position of the hydrogen atoms, by X-ray absorption fine structure (XAFS) analysis, referencing theoretical simulation results. The XANES (X-ray absorption near edge structure) part of the XAFS spectra for each (Ni, Zr and Nb) K-edge of (Ni0.36Nb0.24Zr0.40)89H11 shows clear multiple scattering effects caused by charged hydrogen atoms. The distorted Ni5Zr5Nb3 cluster with the maximum size of 0.55 nm is characterized by five Ni atoms constituting half a pentahedron and an opposite pentahedral Zr-Nb assembly. For the hydrogen localization sites, the first-principles calculations show that there are two Zr-Zr-Nb-Nb, four Zr-Zr-Zr-Nb, two Zr-Zr-Nb-Ni and two Zr-Zr-Zr-Ni tetrahedrons, in the order of stability.