The impact of boron atoms on clathrate-I silicides: composition range of the borosilicide K8-xBySi46-y

The clathrate-I borosilicide K8-xBySi46-y (0.8 <= x <= 1.2 and 6.4 <= y <= 7.2; space group Pm3n) was prepared in sealed tantalum ampoules between 900 degrees C and 1000 degrees C. By high-pressure preparation at 8 GPa and 1000 degrees C, a higher boron content is achieved (x = 0.2, y = 7.8). Crystal structure and composition were established from X-ray diffraction data, chemical analysis, WDX spectroscopy, and confirmed by B-11 and Si-29 NMR, and magnetic susceptibility measurements. The compositions are electron-balanced according to the Zintl rule within one estimated standard deviation. The lattice parameter varies with composition from a = 9.905 angstrom for K7.85(2)B7.8(1)Si38.2(1) to a = 9.968(1) angstrom for K6.80(2)B6.4(5)Si39.6(5)