CRYSTAL-STRUCTURE OF YB41SI1.2

The crystal structure of a new compound, YB41Si1.2, has been determine d by single-crystal X-ray diffractometry. The crystal was grown by the floating-zone method, The intensity data were collected on a four-cir cle diffractometer using CuKalpha radiation, The structure was solved by direct methods and refined with the aid of a full-matrix least-squa res program to the R(R-w) value of 0.034 (0.040); 2686 independent ref lections were used for 232 variable parameters, The structure of YB41S i1.2 belongs to the orthorhombic system (space group Pbam), with latti ce constants a = 16.674(1) Angstrom, b = 17.667(1) Angstrom, and c = 9 .5110(7) Angstrom, and is composed of B-12 icosahedra and a new B12Si3 polyhedral unit. Three Si sites of the B12Si3 unit are assumed to be fully occupied by both Si and B atoms, and the actual chemical composi tion of this unit is estimated to be B12Si1.5B1.5. The boron framework is thus proved to be of a new structure type which is made up of five crystallographically different five B-12 icosahedra and the B12Si3 un it. The Y atom occupies a large hole outside the icosahedra and the B1 2Si3 unit. Interstitial sites, one Si and seven B sites, fire also fou nd outside the polyhedral units, with various occupancies from 100% to 9%. (C) 1997 Academic Press.