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 degrees C. The crystal structure was determined by X-ray diffraction (P-4n2, a = 8.9391(13)angstrom, c = 5.0745(10)angstrom, Z = 2, 713 reflections, 64 variables, R-1(F) = 0.0235, wR(2)(I) = 0.0591). It is closely related to "tetragonal boron Gamma' and can be described as a tetragonal rod packing of corner-linked B-12 icosahedra with C and Mg atoms in the voids. Each B-12 icosahedron has 2 B-C bonds and 10 exohedral bonds to other icosabedra, 2 within the rod and 4 x 2 to neighbouring rods. The isolated C atoms are 4-fold coordinated forming distorted tetrahedra. Mg is placed on two crystal] ographically independent positions within the three-dimensional B12C network. Mg2B24C is the first example for a compound related to "tetragonal boron T' with a stoichiometric composition. (C) 2006 Elsevier Inc. All rights reserved.