New examples of ternary rare-earth metal boride carbides containing finite boron-carbon chains:: The crystal and electronic structure of RE15B6C20 (RE = Pr, Nd)

The ternary rare-earth metal boride carbides RE15B6C20 (RE = Pr, Nd) were synthesized by co-melting the elements. They exist above 1270K. Their crystal structures were determined from single-crystal X-ray diffraction data. Both crystallize in the space group P (1) over bar, Z= 1, a = 8.3431(8) angstrom, b = 9.2492(9) angstrom, c = 8.3581(8) angstrom, alpha = 84.72(1)degrees, beta = 89.68(1)degrees; gamma = 84.23(1)degrees (R1 = 0.041 (wR2 = 0.10) for 3291 reflections with I-o > 2 sigma 1(I-o)) for Pr15B6C20, and a = 8.284(1)angstrom, b = 9.228( 1)angstrom, c = 8.309(1)angstrom, alpha = 84.74(1)degrees, beta = 89.68(1)degrees, gamma = 84.17(2)degrees (R1 = 0.033 (wR2 = 0.049) for 2970 reflections with I-o > 2 sigma(I-o)) for Nd15B6C20. Their structure consists of a three-dimensional framework of rare-earth metal atoms resulting from the stacking of slightly corrugated and distorted square nets, leading to cavities filled with unprecedented B2C4 finite chains, disordered C-3 entities and isolated carbon atoms, respectively. Structural and theoretical analyses suggest the ionic formulation (RE3+)(15)([B2C4](6-))(3) ([C-3](4-))(2)(C4-)(2)center dot 11 (e) over bar. Accordingly, density functional theory calculations indicate that the compounds are metallic. Both structural arguments as well as energy calculations on different boron vs. carbon distributions in the B2C4 chains support the presence of a CBCCBC unit. Pr15B6C18 exhibits antiferromagnetic order at T-N = 7.9K, followed by a meta-magnetic transition above a critical external field B > 0.03T. On the other hand, Nd15B6C18 is a ferromagnet below T-C approximate to 40K. (C) 2008 Elsevier Inc. All rights reserved.