CRYSTAL AND ELECTRONIC-STRUCTURE OF THE NOVEL LAYERED RARE-EARTH-METAL BORIDE CARBIDE GD2B3C2

The crystal structure of Gd2B3C2 has been determined from single-cryst al X-ray counter data. Gd2B3C2 crystallizes in a unique structure type with the orthorhombic space group Cmmm-D2h19, No. 65, Z = 2. The latt ice parameters are a = 0.3445(1), b = 1.3733(3), and c = 0.37107(7) nm . The structure was solved by combined Patterson- difference Fourier m ethods and refined by full-matrix least-squares calculation. R(F) = SI GMA\DELTAF\/SIGMA\F(o)\ = 0.053 for an asymmetric set of 246 independe nt reflections (\F(o)\ > 3sigma(\F(o)\)). Boron atoms in triangular me tal coordination form infinite zigzag chains branched with carbon atom s. Boron atoms are at a distance of d(B1-B1) = 0.1920 nm with a bond a ngle of phi(B1-B1-B1) = 127.6-degrees. Branched carbon atoms in 4-fold rectangular metal coordination are at a distance d(B1-C) = 0.1597 nm from the B-B chain under bond angles phi(C-B1-B1) = 116.2-degrees. The boron chains are linked to a planar two-dimensional boron-carbon netw ork by additional boron atoms forming rather tight bonds with the bran ched carbon atoms at distances of d(B2-C) = 0.1413 nm under bond angle s phi(C-B2-C = 180-degrees. With respect to the structural chemical bu ilding blocks and near neighbors, the crystal structure of Gd2B2[6Gd+C ,2]B[2C,-]C2[4Gd+2B,-] is related to the structure type of YB[6Y+C,2]C [4Y+B,-], which lacks the nonmetal atom linking the carbon-branched bo ron zigzag chains. The electronic structure and bonding properties of Gd2B3C2 are analyzed by means of extended Huckel tight-binding calcula tions. The results show that an ionic picture between the metallic and the boron-carbon sublattices is a good starting point to explain the arrangement observed in the nonmetal framework. Thus, a formal charge of 5-per B3C2 unit accounts for the boron-carbon structural arrangemen t of the B-C net in Gd2B3C2. The electronic structure of the anionic t wo-dimensional boron-carbon layers is found to be strongly related to that of the boron-carbon layers encountered in ThB2C and alpha-UB2C st ructures. The possibility of B-B bond alternation is discussed. In the three-dimensional material, the metal-nonmetal bonding occurs primari ly through electron donation from the anionic sublattice toward the me tallic elements, leading to a metallic behavior.