A study of low-energy guest phonon modes in clathrate-II Na _xSi₁₃₆ (x = 3,23, and 24)

Single-crystal x-ray diffraction from clathrate-II Na_x Si ₁₃₆ (x = 24) prepared by a new technique reveals the exceptionally large Na@Si₂₈ atomic displacement parameter (U_eq) is strongly temperature dependent, and can be attributed to low-energy rattling modes associated with the Na guest. Inelastic neutron scattering (INS) spectra collected from Na_x Si₁₃₆ powder specimens (x = 3, 23) confirm the presence of low-energy guest-derived phonon modes for Na@Si ₂₈ and Na@Si₂₀. The lower energy Na@Si₂₈ rattler mode falls in the frequency range of the silicon host acoustic phonons, indicating the possibility for interaction with these phonons. The presence of these low-energy modes combined with the ability to controllably vary the guest content presents a unique opportunity for exploring the influence of guest-framework interactions on the lattice dynamics in intermetallic clathrates.