Abstract

Two new boron-rich scandium borides were found with a structure closely related to tetragonal boron II (t-B₁₉₂). Some black single crystals of ScB₁₅ were obtained from a gallium melt besides rhombohedral ScB₃₁. Its crystal structure (tP192; P4₁, a = 10.2995(15), c = 14.223(3) Å, 4603 refl., 438 param., R₁(F) = 0.043, wR₂(I) = 0.070) consists of a rod packing of B₁₂-icosahedra and a new polyhedral B₂₀₊₁ unit. Using a Cu-melt the main product is a ternary compound with the composition Sc₂Cu_1−xB₄₅ (x ≈ 0.23) and a very similar crystal structure (tP192; P4₃, a = 10.2674(15), c = 14.261(3) Å, 5440 refl., 461 param., R₁(F) = 0.032, wR₂(I) = 0.080). In Sc₂Cu_1−xB₄₅ one metal position is partially occupied by Cu, resulting in a different charge transfer by the metal atoms. This leads to slight differences in the B₂₀₊₁ unit of the boron framework. Both single crystals were merohedrally twinned. The differences between ScB₁₅ and Sc₂Cu_1−xB₄₅ can be explained by the mnopq-rule of Jemmis.

Graphical abstract