The ternary system Au-Ba-Si: Clathrate solution, electronic structure, physical properties, phase equilibria and crystal structures

We report on (i) the phase relations at 800 °C within the ternary system Au-Ba-Si up to 33.3 at.% Ba, (ii) on the crystallographic data of new ternary compounds, (iii) on details of the clathrate type I solid solution, (iv) on electrical and thermal transport measurements for Ba8Au5.1Si40.9 supported by (v) density functional theory calculations. The clathrate type I solid solution Ba8AuxSi46−x at 800 °C extends from Ba8Au4Si42 (a = 1.039 nm) to Ba8Au6Si40 (a = 1.042 nm). The cubic primitive symmetry (space group Pm3¯n) was confirmed by X-ray powder diffraction in the whole homogeneity region. The lattice parameters of the solid solution show an almost linear increase with increasing gold content and site preferences from X-ray refinement confirm that gold atoms preferably occupy the 6d site in random mixture with Si atoms. The phase equilibria at 800 °C are characterized by seven ternary phases in the investigated region up to 33.3 at.% Ba. The homogeneity range has been established for Ba(Au1−xSix)2 (AlB2 type, extending from BaAu0.4Si1.6 to BaAu0.9Si1.1). BaAu2+xSi2−x (unknown structure type) exhibits a very small homogeneity range (x = 0.6-0.7) and two other ternary phases exist at about 22 at.% Ba, 52 at.% Au and 28 at.% Si and 20 at.% Ba, 58 at.% Au and 22 at.% Si (structure types for both unknown). The crystal structures of two further novel phases in the gold-rich part have been determined from single crystal X-ray data: BaAu3+xSi1−x of BaAu3Ge type (x = 0-0.3, space group P4/nmm, x = 0: a = 0.6488(2), c = 0.5305(2) nm) and BaAu5−xSi2+x (x = 0-0.2, own structure type, space group Pnma, x = 0: a = 0.8935(2), b = 0.6939(2), c = 1.0363(2) nm). The proximity of Ba8AuxSi46−x to a metal to insulator transition is corroborated by density functional theory electronic structure calculations. A gap in the electronic density of states, located near the Fermi energy, gives rise to distinct features of the temperature-dependent electrical resistivity and Seebeck effect.