Phase Equilibria, Crystal Chemistry and Physical Properties of Au-Ba-Ge Clathrates

Abstract

In the Au-Ba-Ge system the clathrate type I solid solution, Ba₈Au _x Ge_46−x−y □ _y , extends at 800 °C from binary Ba₈Ge₄₃□₃ (□ is a vacancy) to Ba₈Au₆Ge₄₀. For the clathrate phase (1 ≤ x ≤ 6) cubic primitive symmetry (space group $Pm{\bar{{3}}}n$) was confirmed by x-ray powder diffraction assisted by x-ray single crystal analyses of Ba₈Au_4.6Ge_40.3□_1.1. The lattice parameters of the solid solution show an almost linear increase with increasing gold content. Site preference from x-ray refinement shows that gold atoms preferably occupy the 6d site in random mixture with Ge and vacancies, which vanish at the solubility limit. Clathrate type ΙX (Ba₆Ge₂₅ type) has a maximum solubility of 2.7 at.% gold at 800 °C. Phase equlilibria at 800 °C are characterized by four ternary phases in the investigated region up to 33.3 at.% barium. The homogeneity range of Ba(Au_1−x Ge _x )₂ (AlB₂-type) and BaAu_1+x Ge_3−x has been established: Ba(Au_1−x Ge _x )₂ extends from BaAu_0.5Ge_1.5 to BaAu_0.9Ge_1.1 and BaAu_1+x Ge_3−x from BaAu_1.1Ge_2.9 (BaNiSn₃-type) to BaAu_2.7Ge_1.3 (Ce(Ni,Sb)₄-type). The crystal structures of two phases in the gold-rich part have been determined from single crystal x-ray data and were found to form new structure types: BaAu₃Ge with BaAu₃Ge-type (space group P4/nmm, a = 0.6459(2), c = 0.5487(2) nm) and BaAu_5+x Ge_2−x (x = 0, BaAu₅Si₂-type, space group Pnma, a = 0.8981(2), b = 0.7106(2) and c = 1.0363(2) nm), the latter revealing with increasing gold content a closely related derivative structure type (BaAu_5.3Ge_1.7, $a = a_{{{\text{BaAu}}_{5} {\text{Si}}_{2} }} ,\;b = b_{{{\text{BaAu}}_{5} {\text{Si}}_{2} }} ,\;c = 2c_{{{\text{BaAu}}_{5} {\text{Si}}{}_{2}}}$). Transport properties and particularly the thermoelectric behavior were studied for Ba₈Au₆Ge₄₀.