Structural relationships between new carbide La 14Sn(MnC 6) 3 and fully ordered La 11(MnC 6) 3

Abstract

Crystals of the ternary La 11(MnC 6) 3 and new quaternary carbide La 14Sn(MnC 6) 3 phases were grown from La/Ni eutectic flux and their structures were determined by means of X-ray single crystal diffraction. La 11(MnC 6) 3 is a new superstructure variant of La 3.67MnC 6 (previously reported disordered subcell: P6 3/ m; a 0=8.806 Å; c 0=5.329 Å, Z=2). The superstructure ( R3¯; a=√3 a 0=15.2649(9) Å; c=3 c 0=16.013(1) Å, Z=6; R 1=0.022) is realized by complete ordering of the La chains within the columns of face-sharing carbon octahedra, with alternating La–La distances leading to R-centering and enlargement of the unit cell. The structure of the quaternary carbide La 14Sn(MnC 6) 3 ( P6¯; a=8.756(1) Å; c=10.483(2) Å, Z=1; R 1=0.026) is closely related to that of La 11(MnC 6) 3 with part of the MnC 6 units replaced by Sn atoms. The structure and precise composition of La 14Sn(MnC 6) 3 can be derived from that of La 11(MnC 6) 3 by taking into account the extent of this substitution and variation in lanthanum siting in the chain of carbon octahedra. Band structure calculations indicate both phases are metallic; the La 11(MnC 6) 3 phase is stabilized by the ordering of La atoms which induces a pseudogap at E F.