Abstract
Single crystals of two modifications of the new magnesium boride carbide MgB~12~C~2~ were synthesized from the elements in a metallic melt by using tantalum ampoules. Crystals were characterized by single‐crystal X‐ray diffraction and electron microprobe analysis (energy‐dispersive (EDX) and wavelength‐dispersive (WDX) X‐ray spectroscopy). Orthorhombic MgB~12~C~2~ is formed in a Cu/Mg melt at 1873 K. The crystal structure of o‐MgB~12~C~2~ (Imma, Z=4, a=5.6133(10), b=9.828(2), c=7.9329(15) Å, 574 reflections, 42 variables, R~1~(F)=0.0208, wR~2~(I)=0.0540) consists of a hexagonal primitive array of B~12~ icosahedra with Mg atoms and C~2~ units in trigonal‐prismatic voids. Each icosahedron has six exohedral BB and six BC bonds. Carbon is tetrahedrally coordinated by three boron atoms and one carbon atom with a remarkably long CC distance of 1.727 Å. Monoclinic MgB~12~C~2~ is formed in an Al/Mg melt at 1573 K. The structure of m‐MgB~12~C~2~ (__C__2/c, Z=4, a=7.2736(11), b=8.7768(13), c=7.2817(11) Å, β=105.33(3)°, 1585 reflections, 71 variables, R~1~(F)=0.0228, wR~2~(I)=0.0610) may be described as a distorted cubic close arrangement of B~12~ icosahedra. Tetrahedral voids are filled by C atoms and octahedral voids are occupied by Mg atoms. The icosahedra are interconnected by four exohedral BB bonds to linear chains and by eight interstitial C atoms to form a three‐dimensional covalent network. Both compounds fulfill the electron‐counting rules of Wade and Longuet‐Higgins.