Different Structural Models of YB_2C_2 and GdB_2C_2 on the Basis of Single-Crystal X-Ray Data

摘要

Samples of YB_2C_2 and GdB_2C_2 were synthesized by arc-melting and subsequent annealing of the products. Single crystals of the title compounds were examined with single-crystal X-ray methods. Five different tetragonal unit cell settings from the literature (obtained on the basis of symmetry considerations) were used for the refinement of the crystal structure of YB_2C_2, but a converging refinement was only achieved for the structural model in the space group P4/mbm (no. 127, Z = 2) with a = 533.27(3) and c = 354.58(3) pm, without obvious inconsistencies. Nevertheless, the results for the five different unit cell settings are compared and discussed. A refinement of the crystal structure of the isotypic compound GdB_2C_2 was also performed with the lattice parameters a = 537.46(6) and c = 364.98(11) pm. Neither EuB_2C_2 nor YbB_2C_2 were obtained by melting coldpressed pellets with the nominal composition RE_(1.5)B_2C_2 in an arc-furnace or an induction furnace, or by just heating the pellets in sealed Ta ampoules.