Rare-earth transition-metal gallium chalcogenides RE3MGaCh7 (M=Fe, Co, Ni; Ch=S, Se)

Six series of quaternary rare-earth transition-metal chalcogenides RE₃MGaCh₇ (M=Fe, Co, Ni; Ch=S, Se), comprising 33 compounds in total, have been prepared by reactions of the elements at 1050 °C (for the sulphides) or 900 °C (for the selenides). They adopt noncentrosymmetric hexagonal structures (ordered Ce₃Al_1.67S₇-type, space group P6₃, Z=2) with cell parameters in the ranges of a=9.5-10.2 Å and c=6.0-6.1 Å for the sulphides and a=10.0-10.5 Å and c=6.3-6.4 Å for the selenides as refined from powder X-ray diffraction data. Single-crystal structures were determined for five members of the sulphide series RE₃FeGaS₇ (RE=La, Pr, Tb) and RE₃CoGaS₇ (RE=La, Tb). The highly anisotropic crystal structures consist of one-dimensional chains of M-centred face-sharing octahedra and stacks of Ga-centred tetrahedra all pointing in the same direction. Magnetic measurements on the sulphides reveal paramagnetic behaviour in some cases and long-range antiferromagnetic behaviour with low Néel temperatures (15 K or lower) in others. Ga L-edge XANES spectra support the presence of highly cationic Ga tetrahedral centres with a tendency towards more covalent Ga-Ch character on proceeding from the sulphides to the selenides. Band structure calculations on La₃FeGaS₇ indicate that the electronic structure is dominated by Fe 3d-based states near the Fermi level.