Structure Refinement of YB 62and YB 56of the YB 66-Type Structure

The crystal structure of YB₆₂and YB₅₆of the YB₆₆-type structure (cubic; space group,Fm3c) has been studied by single-crystal X-ray diffractometry. The crystals were grown by the indirect heating floating-zone method; YB₆₂(a=23.4364(6) Å) was obtained by growth at a congruent composition and YB₅₆(a=23.4600(9) Å) by growth at an incongruent composition. The structure was refined with the aid of a full-matrix least-squares program to theR(R_w) values of 0.069 (0.062) for YB₆₂and 0.075 (0.069) for YB₅₆. The boron framework of the present 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 YB₆₂and YB₅₆, obtained by chemical analysis, there is a significant increase in the yttrium site occupancy; 0.532(4) for YB₆₂and 0.575(5) for YB₅₆. A marked anisotropy of thermal parameters was observed for the Y site, suggesting that disordered accommodation of the Y atom in the "Y-hole" has occurred.