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

Quasielastic neutron scattering (QENS) and neutron powder diffraction of the complex transition metal hydrides Li(5)MoH(11 )and Li6NbH11 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(9)(3- )or NbH94-) 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 beta asymptotic to 0.3-0.4, suggesting a wide relaxation time distribution. The Q dependence of the elastic incoherent structure factor is reproduced by the rotational diffusion of MH9 (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 MH9 rotation is due to the positional disorder of the surrounding Li ions and a unique rotation with MH9 anion deformation (pseudorotation).

Automated summary

The structures and dynamics of Li(5)MoH(11) and Li6NbH11, including the dynamics of hydrogen atoms, were investigated using QENS and neutron powder diffraction. It was found that there is a wide relaxation time distribution for the hydrogen atoms, which is influenced by the positional disorder of surrounding ions and the deformation of MH9 anions.