STRUCTURE REFINEMENT OF YB62 AND YB56 OF THE YB66-TYPE STRUCTURE

The crystal structure of YB62 and YB56 of the YB66-type structure (cub ic; space group, Fm3c) has been studied by single-crystal X-Pay diffra ctometry. The crystals were grown by the indirect heating floating-zon e method; YB62 (a = 23.4364(6) Angstrom) aas obtained by growth at a c ongruent composition and YB56 (a = 23.4600(9) Angstrom) by growth at a n incongruent composition, The structure was refined with the aid of a full-matrix least-squares program to the R (R-w) values of 0.069 (0.0 62) for YB62 and 0.075 (0.069) for YB56. The boron framework of the pr esent crystals is almost identical to that of YB66 (S. M. Richards and J. S. Kasper, 1969, Acta Crystallogr. 25, 237), However, as expected from the chemical compositions of YB62 and YB56, obtained by chemical analysis, there is a significant increase in the yttrium site occupanc y; 0.532(4) for YB62 and 0.575(5) for YB56. A marked anisotropy of the rmal parameters was observed far the Y site, suggesting that disordere d accommodation of the Y atom in the ''Y-hole'' has occurred. (C) 1997 Academic Press.