On the synthesis and crystal structure of BaC2

By the reaction of BaCO3 with graphite in an are melting furnace single crystals of BaC2 could be synthesized. Structural investigations confirm the tetragonal structure (space group: I 4/mmm, Z = 2) given in the literature. In another approach polycrystalline BaC2 of high purity could be prepared by the reaction of elemental barium and graphite at temperatures of about 1000 K. Temperature dependent X-ray powder investigations on this sample confirm the phase transition to a cubic phase (space group: F m (3) over bar m, Z = 4) at about 520 K (1st order phase transition). At low temperatures a new monoclinic form of BaC2 (space group: C 2/c, Z = 4) was found which is isotypic to the crystal structure of ThC2 at room temperature. This structure was refined from neutron TOF diffraction data at 20 K. (C) 1999 Elsevier Science B.V. All rights reserved.