Single crystal studies on Co-containing τ-borides Co23−xMxB6 (M=Al, Ga, Sn, Ti, V, Ir) and the boron-rich τ-boride Co12.3Ir8.9B10.5

Single crystals of the cubic τ-Borides Co23–xMx B6 (M =Al, Ga, Sn) were synthesised from the elements at temperatures between 1200 and 1500 °C. According to the structure refinements one (Ga, Sn: 8c ) or two (Al: 4a and 8c ) of the four independent metal sites show a mixed occupation Co/M resulting in the compositions Co20.9Al2.1B6, Co21.9Ga1.1B6, and Co21.4Sn1.6B6, respectively. Melts with Indium gave access to Co23B6 as the first binary τ-boride (View the MathML sourceFm3¯m,a=10.4618(13) Å, 104 refl., 14 param., R1(F)=0.0132, wR2(F2)=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 Co16.2Ir6.8B6. A higher Ir-content enables the uptake of additional boron resulting in a composition Co12.3Ir8.9B10.5. This can be explained be the substitution of metal atoms on the 8c-site by B4-tetrahedra. A boron-rich phase was observed for the first time for a τ-boride of cobalt. All compositions were confirmed by EDX measurements.