Rare-earth Metal Borosilicides R9Si15-xB3 (R = Tb, Yb): New Ordered Structures Derived from the AlB2 Structure Type

Two novel ternary borosilicides R9Si15-xB3 (R = Tb, x = 1.80, R = Yb, x = 1.17) were synthesized from the initial elements using tin flux method. Their crystal structures were determined by means of X-ray single crystal diffraction. Both refer to space group R32, Z = 1: a = 6.668(2) angstrom, c = 12.405(4) angstrom [R-1 = 0.027, wR(2) = 0.031 for 1832 reflections with I-o > 2 sigma (I-o)] for Tb9Si15-xB3, and a = 6.5796(3) angstrom, c = 12.2599(5) angstrom [R-F = 0.052, wR = 0.090 for 1369 reflections with I-o > 2 sigma (I-o)] for Yb9Si15-xB3. The structures represent a new structure type, derived from that of AlB2, with ordering in the metalloid sublattice resulting in distorted [Si5B] hexagons. The presence or absence of boron in this ordered structure is discussed on the basis of difference Fourier syntheses, interatomic distances, structural analysis, and theoretical calculations in relation with the parent structures of the binaries AlB2 and Yb3Si5 (Th3Pd5 type of structure). Theoretical calculations show substantial covalent interactions between the metal and nonmetal elements. The small percentage of silicon atoms, which are missing in these nonstoichiometric compounds, probably allows strengthening boron-metal and boron-silicon bonding.