Clathrate formation in the Ba-Pd-Ge system: Phase equilibria, crystal structure, and physical properties

摘要

Phase relations at subsolidus temperatures as well as at T=800 ℃, crystallographic data, electrical and thermal transport measurements, and heat capacity data are reported for several compositions within the clathrate type-I solid solution: Ba_8Pd_x-Ge_(46-x-y)□_y (□ is a vacancy). The solid solution derives from binary clathrate Ba_8Ge_(43)□_3 with a solubility limit of 3.8 Pd atoms per formula unit at T=800 ℃. Structural investigations throughout the homogeneity region confirm cubic primitive symmetry consistent with the space group type Pm3n and lattice parameters ranging from a =1.0657(2) nm for Ba_8Ge_(43)□_3 to a =1.077 41(2) nm for Ba_8Pd_(3.8)Ge_(42.2)D_(0.0) The primary field of clathrate crystallization has been elucidated from micrography and differential thermal analyses. Both heat capacity and inelastic neutron diffraction define a low-lying, almost localized, phonon branch. Studies of transport properties evidence electrons as the majority charge carriers for most of the homogeneity region; however, at the Pd-rich limit, holes dominate the electronic transport. The crossover between both regimes provides appropriate conditions for attractively high Seebeck values. The lattice contribution dominates the overall thermal conductivity.