R(Au∼0.75Sb∼0.25)2 (R = La, Ce, Pr) with UHg2 structure type, new members of the AlB2 family

The crystal structure of a new series of ternary rare-earth gold antimonides R(Au0.75Sb0.25)2 (R=La, Ce, Pr) has been studied by X-ray powder diffraction from the 'as-cast' alloys. The hexagonal UHg2 structure type, a deformation derivative of AlB2 structure type, space group P6/mmm (No. 191), has been found for all compounds. Rietveld refinements for all three compounds were performed. The coordination polyhedron of R atom is hexagonal prism formed by 12 atoms of X (statistical mixture of Au and Sb atoms) with 2 additional atoms of R against base faces: [RX12R2]. The coordination polyhedron of statistical mixture X has the shape of full-capped trigonal prism [XR6X3X2].