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

In the Au-Ba-Ge system the clathrate type I solid solution, Ba(8)AuxGe(46-x-y)square(y), extends at 800 degrees C from binary Ba8Ge43 square(3) (square is a vacancy) to Ba8Au6Ge40. For the clathrate phase (1 <= x <= 6) cubic primitive symmetry (space group Pm (3) over barn) was confirmed by x-ray powder diffraction assisted by x-ray single crystal analyses of Ba8Au4.6Ge40.3 square(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 IX (Ba6Ge25 type) has a maximum solubility of 2.7 at.% gold at 800 degrees C. Phase equlilibria at 800 degrees C are characterized by four ternary phases in the investigated region up to 33.3 at.% barium. The homogeneity range of Ba(Au1-xGex)(2) (AlB2-type) and BaAu1+xGe3-x has been established: Ba(Au1-2xGex)(2) extends from BaAu0.5Ge1.5 to BaAu0.9Ge1.1 and BaAu1+ xGe3-x from BaAu1.1Ge2.9 (BaNiSn3-type) to BaAu2.7Ge1.3 (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 BaAu5+ xGe2-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 (BaAu5.3Ge1.7, a = a(BaAu5Si2); b = b(BaAu5Si2); c = 2c(BaAu5Si2)). Transport properties and particularly the thermoelectric behavior were studied for Ba8Au6Ge40.