Single crystal studies on boron-rich τ-borides Ni23-xMxB6 (M=Zn, Ga, In, Sn, Ir)-The surprising occurrence of B4-tetraheda as a normal case?

Single crystals of the cubic τ-borides Ni23-xMxB6 (M=Zn, Ga, In, Sn, Ir) were synthesised from the elements at temperatures between 1200 and 1500 oC. The structure refinements show that the existence of boron-rich phases is quite common. Starting from the idealised composition Ni20M'3B6 a part of the metal atoms on site 8c is substituted by B4 tetrahedra. For M'=Ga a complete exchange seems to be possible leading to the composition Ni20GaB14. For M'=Zn and Sn the formation of solid solutions is less extended. For M'=In no exchange is observed but an unusual pattern of Ni/In distribution is observed. With M=Ir mixed occupations occur for all sites and the boron content varies, too. All compositions were confirmed by EDX measurements. - Graphical abstract: Crystal structure of τ-Borides M23B6; M1: M8-cubes, M2: M12-cuboctahedra centred by M3, isolated M-atoms: M4; grey circles: boron, black circles: metal atoms.