On the boron rich phases in the Yb-B system

Two boron rich phases were successfully synthesized by borothermal reduction of Yb oxide under vacuum. For the new boron-poorer phase, the single phase was established at around [B]/[Yb] = 43.3 at 1500 °C (Pbam space group; YB ₅₀-type; a = 16.5811(5) Å, b = 17.5950(5) Å, c = 9.4647(3) Å; powder X-ray diffraction; Rietveld refinement). The crystal structure of the boron-richer phase ([B]/[Yb] = 56.7) has been elucidated by single crystal X-ray diffraction (Fm3¯c space group; YB ₆₆-type; a = 23.3587(6) Å). Powder X-ray diffraction data of the alloy YbB _~70 annealed at 1825 °C yielded, along with the YB ₆₆-type compound (a = 23.3691(2) Å), β-rh B as a secondary phase (R3¯m space group, a = 10.9298(3) Å, c = 23.875(1) Å), for which the solubility of Yb was found to be below 1 at%. The Yb atoms in β-rh B occupy the D and E voids. Both YbB _43.3 (YB ₅₀-type) and YbB _56.7 (YB ₆₆-type) feature complicated boron atom frameworks which exhibit shorter B-B separations both within and between boron clusters as compared to those observed for prototype structures.