Structure and dynamics of empty cages in xenon clathrate hydrate

We performed molecular dynamics calculations of xenon clathrate hydrate to investigate the effects of empty cages on the structure and dynamics of the surrounding lattice. The distinct structure and dynamics of the empty cages, and cages including Xe, which coexist in the lattice, were analyzed. The results show that the ellipsoidal tetrakaidecahedral cage shrinks along the minor 〈 100 〉 axis and expands along the major 〈 100 〉 axis due to the absence of Xe from the cage, whereas the dodecahedral cage shrinks isotropically. These distortions of the empty cages cause a reduction in the lattice constant and an enhancement of the thermal vibrations of the surrounding lattice. The vibrational density of states shows that the hydrogen bonds consisting of the tetrakaidecahedral cage are strengthened by the absence of Xe, whereas those of the dodecahedral cage are weakened. These results show differing mechanisms of guest-host interaction for the two types of cages including Xe. Repulsion is the dominant guest-host interaction for the dodecahedral cage, as proposed by previous studies. For the tetrakaidecahedral cage, however, attractive interaction is dominant along the major 〈 100 〉 axis, whereas repulsive interaction is dominant along the minor 〈 100 〉 axis. The present results suggest that a small number of empty cages can affect not only the local structures but also the macroscopic properties of the crystal. It is concluded that the distortions of the empty cages are one of the important factors governing the density and phase equilibrium of clathrate hydrates.