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

Abstract

Phase relations at subsolidus temperatures as well as at T=800 °C, crystallographic data, electrical and thermal transport measurements, and heat capacity data are reported for several compositions within the clathrate type-I solid solution: Ba8PdxGe46−x−y y ( is a vacancy). The solid solution derives from binary clathrate Ba8Ge43 3 with a solubility limit of 3.8 Pd atoms per formula unit at T=800 °C. Structural investigations throughout the homogeneity region confirm cubic primitive symmetry consistent with the space group type Pmn and lattice parameters ranging from a=1.0657(2) nm for Ba8Ge433 to a=1.077 41(2) nm for Ba8Pd3.8Ge42.2 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.