Synthesis and crystal structure of Mg2B24C, a new boron-rich boride related to “tetragonal boron I”

Single crystals of Mg2B24C, a new boron-rich boridecarbide of magnesium, were synthesized as black needles and columns by reaction of the elements in Ta ampoules and BN crucibles at 1300 °C. The crystal structure was determined by X-ray diffraction (P-4n2, image, image, image, 713 reflections, 64 variables, image, image). It is closely related to “tetragonal boron I” and can be described as a tetragonal rod packing of corner-linked B12 icosahedra with C and Mg atoms in the voids. Each B12 icosahedron has 2 B–C bonds and 10 exohedral bonds to other icosahedra, 2 within the rod and 4×2 to neighbouring rods. The isolated C atoms are 4-fold coordinated forming distorted tetrahedra. Mg is placed on two crystallographically independent positions within the three-dimensional B12C network. Mg2B24C is the first example for a compound related to “tetragonal boron I” with a stoichiometric composition.