Structure refinement of YB62 and YB56 of the YB66-type structure

The crystal structure of YB62 and YB56 of the YB66-type structure (cubic; space group, Fm3c) has been studied by single-crystal X-Pay diffractometry. The crystals were grown by the indirect heating floating-zone method; YB62 (a = 23.4364(6) Angstrom) aas obtained by growth at a congruent composition and YB56 (a = 23.4600(9) Angstrom) by growth at an 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 (0.062) for YB62 and 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 YB62 and YB56, obtained by chemical analysis, there is a significant increase in the yttrium site occupancy; 0.532(4) for YB62 and 0.575(5) for YB56. A marked anisotropy of thermal parameters was observed far the Y site, suggesting that disordered accommodation of the Y atom in the ''Y-hole'' has occurred. (C) 1997 Academic Press.