Crystal Structure of Novel Ni–Zn Borides: First Observation of a Boron–Metal Nested Cage Unit: B20Ni6

The crystal structures of three ternary Ni–Zn borides have been elucidated by means of X-ray single-crystal diffraction (XSC) and X-ray powder diffraction techniques (XPD) in combination with electron microprobe analyses (EMPA) defining the Ni/Zn ratio. Ni21Zn2B24 crystallizes in a unique structure type (space group I4/mmm; a = 0.72103(1) nm and c = 1.42842(5) nm; RF2 = 0.017), which contains characteristic isolated cages of B20 units composed of two corrugated octogonal boron rings, which are linked at four positions via boron atoms. The B20 units appear to have eight-membered rings on all six faces like the faces of a cube. Each face is centered by a nickel atom. The six nickel atoms are arranged in the form of an octahedron nested within the B20 unit. Such a boron aggregation is unique and has never been encountered before in metal–boron chemistry. The crystal structure of Ni12ZnB8–x (x = 0.43; space group Cmca, a = 1.05270(2) nm, b = 1.45236(3) nm, c = 1.45537(3) nm; RF2 = 0.028) adopts the structure type of Ni12AlB8 with finite zigzag chains of five boron atoms. The compound Ni3ZnB2 crystallizes in a unique structure type (space group C2/m, a = 0.95101(4) nm, b = 0.28921(4) nm, c = 0.84366(3) nm, β = 101.097(3)°, and RF2 = 0.020) characterized by B4 zigzag chain fragments with B–B bond lengths of 0.183–0.185 nm. The Ni3ZnB2 structure is related to the Dy3Ni2 type.