title: A study of low-energy guest phonon modes in clathrate-II Na(x)Si136 (x = 3, 23, and 24).
abstract: Single-crystal x-ray diffraction from clathrate-II Na(x)Si(136) (x = 24) prepared by a new technique reveals the exceptionally large Na@Si(28) 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)Si(136) powder specimens (x = 3, 23) confirm the presence of low-energy guest-derived phonon modes for Na@Si(28) and Na@Si(20). The lower energy Na@Si(28) 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.
authors.affiliation: Department of Physics, University of South Florida, Tampa, FL 33620, USA.
authors.forename: M
authors.initials: M
authors.lastname: Beekman
authors.forename: R P
authors.initials: RP
authors.lastname: Hermann
authors.forename: A
authors.initials: A
authors.lastname: Möchel
authors.forename: F
authors.initials: F
authors.lastname: Juranyi
authors.forename: G S
authors.initials: GS
authors.lastname: Nolas