A novel boroncarbide, Y2B3C2. X-ray single crystal and neutron powder diffraction analysis

The crystal structure of Y2B3C2 was analysed by means of room temperature X-ray single crystal and neutron powder diffractometry. Y2B3C2 crystallises with the Gd2B3C2-type (space group: Cmmm - No. 65, Z=2). The unit cell dimensions are: a=0.3405(5), b=1.3765(6), c=0.3631(6) nm, b/a=4.044, V=0.1702 nm3 the X-ray density is: ρx=4.57 Mgm-3. The structure was solved by combined Patterson-Difference Fourier methods. Residual values are: RF=Σ=0.027, Rw=0.020, for an asymmetric set of 232 independent reflections (3σ()). Boron atoms B1, each in triangular metal co-ordination, form infinite zigzag chains (dB1-B1=0.1923 nm) branched with carbon atoms. Bond angles in the chain are φB1-B1-B1=124.6 . Carbon atoms are at a distance of dB1-C=0.1552 in rectangular co-ordination of four metal atoms. The branched boron chains are connected by a bridging boron atom B2 linking the carbon atoms in form of a linear bridge at dC-B2=0.1442 nm. Rietveld refinement of the neutron powder data from Y2B3C2(11B-isotope) confirms the nuclear structure arrangement as well as the full occupancy of all the non-metal atom sites. (orig.)