Synthesis, crystal structure and properties of Mg{sub 3}B{sub 36}Si{sub 9}C and related rare earth compounds RE{sub 3−x}B{sub 36}Si{sub 9}C (RE=Y, Gd–Lu)

We report on the synthesis and characterisation of Mg{sub 3}B{sub 36}Si{sub 9}C. Black single crystals of hexagonal shape were yielded from the elements at 1600 °C in h-BN crucibles welded in Ta ampoules. The crystal structure (space group R3{sup ¯}m, a=10.0793(13) Å, c=16.372(3) Å, 660 refl., 51 param., R{sub 1}(F)=0.019; wR{sub 2}(F{sup 2})=0.051) is characterized by a Kagome-net of B{sub 12} icosahedra, ethane like Si{sub 8}-units and disordered SiC-dumbbells. Vibrational spectra show typical features of boron-rich borides and Zintl phases. Mg{sub 3}B{sub 36}Si{sub 9}C is stable against HF/HNO{sub 3} and conc. NaOH. The micro-hardness is 17.0 GPa (Vickers) and 14.5 GPa (Knoop), respectively. According to simple electron counting rules Mg{sub 3}B{sub 36}Si{sub 9}C is an electron precise compound. Band structure calculations reveal a band gap of 1.0 eV in agreement to the black colour. Interatomic distances obtained from the refinement of X-ray data are biased and falsified by the disorder of the SiC-dumbbell. The most evident structural parameters were obtained by relaxation calculation. Composition and carbon content were confirmed by WDX measurements. The small but significant carbon content is necessary by structural reasons and frequently caused by contaminations. The rare earth compounds RE{sub 3−x}B{sub 36}Si{sub 9}C (RE=Y, Dy–Lu) are isotypic. Single crystals were grown from a silicon melt and their structures refined. The partial occupation of the RE-sites fits to the requirements of an electron-precise composition. According to the displacement parameters a relaxation should be applied to obtain correct structural parameters. - Graphical abstract: Single crystals of the new boridesilicide Mg{sub 3}B{sub 36}Si{sub 9}C were obtained from the elements in a Si-melt. Besides B{sub 12}-icosahedra and ethan-like Si{sub 8}-units it contains a disordered SiC-dumbbell. Correct distances were obtained by relaxation calculation based on the X-ray data. Highlights: • Black single crystals of Mg{sub 3}B{sub 36}Si{sub 9}C were yielded from the elements at 1600 °C. • The rare earth compounds RE{sub 3–x}B{sub 36}Si{sub 9}C (RE=Y, Gd–Lu) are isotypic. • Correct structural parameters result from X-ray data and subsequent relaxation.