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

Abstract

Quasielastic neutron scattering (QENS) and neutron powder diffraction of the complex transition metal hydrides Li_{5}MoH_{11} and Li_{6}NbH_{11} 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 NbH_{9}^{4−}) 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 β ≈ 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 MH_{9} (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_{9} rotation is due to the positional disorder of the surrounding Li ions and a unique rotation with MH_{9} anion deformation (pseudorotation).