New examples of ternary rare-earth metal boride carbides containing finite boron–carbon chains: The crystal and electronic structure of RE15B 6C 20 ( RE=Pr, Nd)

Abstract

The ternary rare-earth metal boride carbides RE 15B 6C 20 ( RE=Pr, Nd) were synthesized by co-melting the elements. They exist above 1270 K. Their crystal structures were determined from single-crystal X-ray diffraction data. Both crystallize in the space group P1¯, Z=1, a=8.3431(8) Å, b=9.2492(9) Å, c=8.3581(8) Å, α=84.72(1)°, β=89.68(1)°, γ =84.23(1)° ( R1=0.041 (w R2=0.10) for 3291 reflections with I o>2 σ( I o)) for Pr 15B 6C 20, and a=8.284(1) Å, b=9.228(1) Å, c=8.309(1) Å, α=84.74(1)°, β=89.68(1)°, γ=84.17(2)° ( R1=0.033 (w R2=0.049) for 2970 reflections with I o>2 σ( I o)) for Nd 15B 6C 20. 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 B 2C 4 finite chains, disordered C 3 entities and isolated carbon atoms, respectively. Structural and theoretical analyses suggest the ionic formulation ( RE 3+) 15([B 2C 4] 6−) 3([C 3] 4−) 2(C 4−) 2·11ē. 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 B 2C 4 chains support the presence of a CBCCBC unit. Pr 15B 6C 18 exhibits antiferromagnetic order at T N=7.9 K, followed by a meta-magnetic transition above a critical external field B>0.03 T. On the other hand, Nd 15B 6C 18 is a ferromagnet below T C≈40 K.