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 °C. 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.