Rotation of complex ions with ninefold hydrogen coordination studied by quasielastic neutron scattering and first-principles molecular dynamics calculations

Omasa, Yoshinori*; Takagi, Shigeyuki*; Toshima, Kento*; Yokoyama, Kaito*; Endo, Wataru*; Orimo, Shinichi*; Saito, Hiroyuki*; Yamada, Takeshi*; Kawakita, Yukinobu  ; Ikeda, Kazutaka*; Otomo, Toshiya*; Akiba, Hiroshi*; Yamamuro, Osamu*

Quasielastic neutron scattering (QENS) and neutron powder diffraction of the complex transition metalhydrides LiMoH and LiNbH were measured in a temperature range of 10-300 K to study their structures and dynamics, especially the dynamics of the hydrogen atoms. These hydrides contain unusual ninefold H coordinated complex ions (MoH or NbH) and hydride ions (H). A QENS signal appeared 150 K due to the relaxation of H atoms. The intermediate scattering functions derived from the QENS spectra are well fitted by a stretched exponential function called the Kohlrausch-Williams-Watts functions with a small stretching exponent 0.3-0.4, suggesting a wide relaxation time distribution. The dependence of the elastic incoherent structure factor is reproduced by the rotational diffusion of MH (M = Mo or Nb) anions. The results are well supported by a van Hove analysis for the motion of H atoms obtained using first-principles molecular dynamics calculations. We conclude that the wide relaxation time distribution of the MH rotation is due to the positional disorder of the surrounding Li ions and a unique rotation with MH anion deformation (pseudorotation).