Crystal Structure of YbCu6In6 and Mixed Valence Behavior of Yb in YbCu6–xIn6+x (x = 0, 1, and 2) Solid Solution

High quality single crystals of YbCu6In6 have been grown using the flux method and characterized by means of single crystal X-ray diffraction data. YbCu6In6 crystallizes in the CeMn4Al8 structure type, tetragonal space group I4/mmm, and the lattice constants are a = b = 9.2200(13) Å and c = 5.3976(11) Å. The crystal structure of YbCu6In6 is composed of pseudo-Frank–Kasper cages filled with one ytterbium atom in each ring. The neighboring cages share corners along [100] and [010] to build the three-dimensional network. YbCu6–xIn6+x (x = 0, 1, and 2) solid solution compounds were obtained from high frequency induction heating and characterized using powder X-ray diffraction. The magnetic susceptibilities of YbCu6–xIn6+x (x = 0, 1, and 2) were investigated in the temperature range 2–300 K and showed Curie–Weiss law behavior above 50 K, and the experimentally measured magnetic moment indicates mixed valent ytterbium. A deviation in inverse susceptibility data at 200 K suggests a valence transition from Yb2+ to Yb3+ as the temperature decreases. An increase in doping of Cu at the Al2 position enhances the disorder in the system and enhancement in the trivalent nature of Yb. Electrical conductivity measurements show that all compounds are of a metallic nature.