New members of ternary rare-earth metal boride carbides containing finite boron–carbon chains: RE25B 14C 26 ( RE=Pr, Nd) and Nd 25B 12C 28

Abstract

New ternary rare-earth metal boride carbides RE 25B 14C 26 ( RE=Pr, Nd) and Nd 25B 12C 28 were synthesized by co-melting the elements. Nd 25B 12C 28 is stable up to 1440 K. RE 25B 14C 26 ( RE=Pr, Nd) exist above 1270 K. The crystal structures were investigated by means of single-crystal X-ray diffraction. Nd 25B 12C 28: space group P 1 ¯ , a=8.3209(7) Å, b=8.3231(6) Å, c=29.888(2) Å, α=83.730(9)°, β=83.294(9)°, γ=89.764(9)°. Pr 25B 14C 26: space group P2 1/ c, a=8.4243(5) Å, b=8.4095(6) Å, c=30.828(1) Å, β=105.879(4)°, V=2100.6(2) Å 3, ( R1=0.048 (w R2=0.088) from 2961 reflections with I o>2σ( I o)); for Nd 25B 14C 26 space group P2 1/ c, Z=2, a=8.3404(6) Å, b=8.3096(6) Å, c=30.599(2) Å, β=106.065(1)°. Their structures consist 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 cumulene-like molecules [B 2C 4] 6− and [B 3C 3] 7−, nearly linear [BC 2] 5− and bent [BC 2] 7− units and isolated carbon atoms. Structural and theoretical analysis suggests the ionic formulation for RE 25B 14C 26: ( RE 3+) 25[B 2C 4] 6−([B 3C 3] 7−) 2([BC 2] 5−) 4([BC 2] 7−) 2(C 4−) 4·5e − and for Nd 25B 12C 28: (Nd 3+) 25([B 2C 4] 6−) 3([BC 2] 5−) 4([BC 2] 7−) 2(C 4−) 4·7e −. Accordingly, extended Hückel tight-binding calculations indicate that the compounds are metallic in character.