Combined X-ray and neutron diffraction study of vacancies and disorder in the dimorphic clathrate Ba₈Ga₁₆Sn₃₀ of type i and VIII

We report detailed structural investigations of the dimorphic clathrate Ba₈Ga₁₆Sn₃₀ that crystallizes in both type I and VIII clathrate structures. Single crystals of type I and VIII have been examined using single crystal X-ray and Laue neutron diffraction in the temperature range T = 10 K-500 K. The utilization of both X-ray and neutron diffraction gives a unique ability to reveal the occurrence of minute vacancy occupancies in the host structure. The vacancies are shown to be located on the 6c (type I) and 24g (type VIII) framework sites. Largest vacancy densities are observed for type I p-Ba₈Ga₁₆Sn₃₀, 1.3(4)%, and type VIII n-Ba₈Ga₁₆Sn₃₀, 0.7(2)%. The relation between guest atom disorder and occurrence of glasslike thermal conductivity in intermetallic clathrates was also investigated. In type VIII Ba ₈Ga₁₆Sn₃₀ neither n-type (crystalline thermal conductivity) nor p-type (glasslike thermal conductivity) showed any significant disorder of the guest atoms; they do however show anharmonic motion. The glasslike thermal conductivity of p-type Ba₈Ga₁₆Sn ₃₀ is interpretable as a result of higher effective mass of p-type charge-carriers affecting phonon scattering. In type I Ba₈Ga ₁₆Sn₃₀ guest atoms are highly disordered for both carrier types and samples of both charge carrier types have glasslike thermal conductivity.