Single crystal studies on Co-containing τ-borides Co 23−xM xB 6 ( M=Al, Ga, Sn, Ti, V, Ir) and the boron-rich τ-boride Co 12.3Ir 8.9B 10.5

Abstract

Single crystals of the cubic τ-Borides Co 23– xM x B 6 ( M=Al, Ga, Sn) were synthesised from the elements at temperatures between 1200 and 1500 °C. According to the structure refinements one (Ga, Sn: 8 c) or two (Al: 4 a and 8 c) of the four independent metal sites show a mixed occupation Co/ M resulting in the compositions Co 20.9Al 2.1B 6, Co 21.9Ga 1.1B 6, and Co 21.4Sn 1.6B 6, respectively. Melts with Indium gave access to Co 23B 6 as the first binary τ-boride ( Fm 3 ¯ m , a=10.4618(13) Å, 104 refl., 14 param., R 1( F)=0.0132, w R 2( F 2)=0.0210). With M=Ir mixed occupations occur for all sites and the boron content varies. The composition for the boron-poor single crystal was Co 16.2Ir 6.8B 6. A higher Ir-content enables the uptake of additional boron resulting in a composition Co 12.3Ir 8.9B 10.5. This can be explained be the substitution of metal atoms on the 8 c-site by B 4-tetrahedra. A boron-rich phase was observed for the first time for a τ-boride of cobalt. All compositions were confirmed by EDX measurements.