The Crystal Data of Ternary Rare Earth Borides, RCo_2B_2

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Compounds with the composition of RCo_2B_2 (R=La, Nd, Sm, Gd, Tb, Dy, and Y) were prepared by arc-melting methods. Their crystal structure was investigated by means of X-ray diffraction. These ternary rare earth borides crystallize in the tetragonal lattice. The lattice parameters are a=3.616±0.003 A and c=10.215±0.005 A for LaCo_2B_2 and a=3.561±0.002 A and c=9.358±0.005 A for YCo_2B_2. The good agreement between the X-ray diffraction intensities observed and those calculated shows that the ternary borides, LaCo_2B_2 and YCo_2-B_2, crystallize in the ThCr_2Si_2-type structure. The crystallographic data obtained for LaCo_2B_2 and YCo_2B_2 are as follows : space group 14/mmm (D^<17>_<4h>); R in 2(a), 4Co in 4(d), and 4B in 4(e) with z～3/8. The boron atoms in this structure are situated at the center of a trigonal prism formed by four rare earth atoms and two cobalt atoms. We also found the RCo_2B_2 compounds to be isostructural with LaCo_2B_2 and YCo_2B_2, where R=Nd, Sm, Gd, Tb, and Dy. However, efforts to prepare CeCo_2B_2 and ErCo_2B_2 by arc-melting were unsuccessful.

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