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: Ba 8 Pd x Ge 46−x−y ᮀ y ͑ᮀ is a vacancy͒. The solid solution derives from binary clathrate Ba 8 Ge 43 ᮀ 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 Pm3n and lattice parameters ranging from a = 1.0657͑2͒ nm for Ba 8 Ge 43 ᮀ 3 to a = 1.077 41͑2͒ nm for Ba 8 Pd 3.8 Ge 42.2 ᮀ 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.

Structure and Physical Properties of Clathrate I Systems Ba8PdxSi46-xand Ba8PtxSi46-x

“…Consequently, a reduced Debye temperature, associated with the higher Ge mass, naturally explains the experimental findings. Such a conclusion appears to be corroborated by measurements of the thermal conductivity of Ba 8 M x Ge 46−x−y  y (M = Pd, Pt, Zn, Cd), [2][3][4][5] where λ ph (T ) of e.g., the Cd-based materials exhibit distinct lower values than comparable Zn-based samples.…”

“…As a result both the metallic and the semiconducting features are accounted for. [2][3][4][5] The metallic behaviour derives from scattering of conduction electrons into unoccupied states in reciprocal space just above the Fermi energy. Once this region of the DOS becomes occupied, electrons have to be promoted across the gap, originating in a semiconducting behaviour.…”

“…Recent work on Ba-and Ge-based clathrate systems Ba 8 M x Ge 46−x−y  y (M = Au, Pd, Pt, Zn, Cd), [1][2][3][4][5] where M-atoms substitute for framework atoms, has demonstrated that increasing M-content reduces the concentration of vacancies in the crystal structure and drives the metallic system towards a metal-to-insulator transition, yielding interesting thermoelectric behaviour. The present paper extends our systematic investigation towards ternary silicon-based clathrate phases in combinations with Ba and M = Pd, Pt.…”

Single-Crystal Investigations on Quaternary Clathrates Ba8Cu5Si x Ge41−x (x = 6, 18, 41)

“…A number of investigations have been performed to elucidate the fundamental mechanism of the low thermal conductivity in clathrates by either varying the guest atoms [7][8][9][10][11][12] or changing the environment of the guest atoms by substitution of host atoms. 9,11,[13][14][15][16][17][18][19][20][21][22][23][24] Herein, we report our investigations on the type I clathrates Ba 8 Cu 5 Si x Ge 41Àx (x = 6, 18, 41) with the aim of clarifying the structural details and their variation with x, by single-crystal x-ray diffraction.…”

Giant Thermopower at Low Temperatures in Novel Clathrates Ba8{Cu,Zn} x Ge46−x

“…In the simplest model, the electronic structure of clathrates can be understood in terms of the Zintl concept: Binary Ba 8 Ge 43 ( 3 with three framework defects can be formulated as ½Ba 2þ 8 ½Ge 0 43 ½( 4À 3 , yielding an excess of four electrons per formula unit. This simple concept was proven successful in previous substitution and doping studies carried out on binary Ba 8 Ge 43 ( 3 : [3][4][5][6][7][8][9] Substitution and doping turned out to be a fruitful and promising tool to arrange the charge carrier concentration of clathrates in an optimal manner to tailor the highest possible thermoelectric performance. 10 The aim of the present work is to study the transport properties of type I clathrates Ba 8 Ge 43 ( 3 where the charge carrier density is fine-tuned by substituting Ge by Cu, Zn, Pd, and Si.…”