Synthesis and crystal structure of La^21Cr8-2a Al^{b Ge}7-b C 12 [a = 0.22 (2) and b = 0.758 (19)]

Single crystals of a new multinary chromium carbide, La21Cr8-2a Al b Ge7-b C12 (henicosalanthanum octachromium aluminium hexagermanium dodecacarbide), were grown from an La-rich self flux and were characterized by single-crystal X-ray diffraction. The face-centered cubic crystal structure is composed of isolated and geometrically frustrated regular Cr tetrahedra that are co-centered within regular C octahedra. These mutually separated Cr4-aC6 clusters are distributed throughout a three-dimensional framework of Al, Ge, and La. The title compound is isotypic with La21-δMn8 X 7C12 and R 21Fe8 X 7C12 (R = La, Ce, Pr; X = Al, Bi, Ge, Sn, Sb, Te) and represents the first example of a Cr-based compound with this structure-type.