Phonon density of states in different clathrate hydrates measured by inelastic neutron scattering

Clathrate hydrates, depending on the guest molecule type, generally exhibit one out of three different crystal structures: two cubic structures, sI and sII and one hexagonal structure, sH. In the past, our inelastic neutron scattering measurements on hydrogen clathrates have provided information on the quantum dynamics of the guest molecules in the water cages. Besides the guest dynamics, the dynamics of the water lattice itself has a large interest, due to the analogy with ice (e.g. proton disorder), and to the existence of various possible structures. Additionally, in these inclusion compounds, a coupling between the host and the guest motions is generally observed, and is considered to be relevant to explain the anomalous features of some macroscopic properties, such as thermal conductivity. Here, we present a systematic study of the H-projected phonon density of states (H-PDoS) of the lattice modes in clathrate hydrates. We have experimentally investigated the three existing structures (i.e. sI, sII, and sH) through inelastic neutron scattering measurements, and we have extracted the acoustic-optic and the librational H-PDoS's. By using proper isotopic substitutions, we have been able to tune the host scattering intensity with respect to the guest one. The studied samples consisted in three clathrates made of light water (namely, simple sI structure with Xe, simple sII structure with fully deuterated THF, and binary sH structure with MTBE and D₂), and two made of heavy water (namely, simple sII structure with Ne, and simple sII structure with fully deuterated THF). The experimental results have been compared with lattice dynamics simulations performed by us.