Phase equilibria in the palladium-rich part of the gallium–palladium system. The crystal structures of Ga3Pd7 and Ga1−xPd2+x

Abstract The phase relations between the palladium-rich phases of the gallium–palladium system are clarified by combining preparative methodologies with powder X-ray diffraction and thermochemical analyses. A modified phase diagram of the palladium-rich part of the system is presented. The structure of Ga 3 Pd 7 , decomposing peritectoidally at 881(2)°C, was determined by powder X-ray diffraction and refined by a Rietveld profile fit. Ga 3 Pd 7 adopts a new structure type with monoclinic space group symmetry: C2/m , Z =2, a =1359.46(2) pm, b =405.510(5) pm, c =544.339(6) pm, β =105.2219(7)°, wR P =0.082. The structure can be seen as a strongly distorted, ordered variant of a respective Ga x Pd 1− x fcc solid solution. It comprises two structural motifs: layers of columns of fused (GaPd 6/2 ) 2 prisms are formally separated by equiatomic GaPd layers. The impact of the composition on the structure of Co 2 Si-type Ga 1− x Pd 2+ x was studied. There exists a symmetry relation between the cubic close-packed arrangement of atoms and the structures of the Co 2 Si-type branch of AB 2 phases with Pearson symbol oP12 and space group symmetry Pnma .