Y2CO3AL9 WITH Y2CO3GA9 TYPE-STRUCTURE - AN INTERGROWTH OF CSCL- AND TH3PD5-TYPE SLABS

The structure of Y2Co3Al9 was determined by the single-crystal diffraction method (lambda(Mo K-alpha) = 0.71073 angstrom, mu = 19.931 mm-1, F(000) = 1104, T = 293 K, wR = 0.037 for 555 unique reflections). This new aluminide has an orthorhombic structure of Y2Co3Ga9 type, oS56, (63) Cmcm - hg2feca, a = 12.740(2) angstrom, b = 7.4635(9) angstrom, c = 9.321(1) angstrom, V = 886.3(2) angstrom 3, Z = 4, M(r) = 597.44, D(x) = 4.478 mg mm-3. The Y2Co3Al9 structure can be interpreted as being built up of two kinds of slabs cut from the simple structures CsCl (composition CoAl) and Th3Pd5 (composition Y2Al6): Co3Al3 + Y2Al6 = Y2Co3Al9. The cobalt atoms are surrounded by eight aluminium atoms at contact distances, forming a distorted cube, and four yttrium atoms at relatively longer distances, the 12 atoms together forming an icosahedron. The yttrium atoms centre capped hexagonal prisms, the coordination numbers for the aluminium atoms are 10 or 11.