Noncentrosymmetric rare-earth copper gallium chalcogenides RE 3CuGaCh 7 (RE=La–Nd; Ch=S, Se): An unexpected combination

The quaternary rare-earth chalcogenides RE 3CuGaS7 and RE 3CuGaSe7 (RE=La–Nd) have been prepared by reactions of the elements at 1050°C and 900°C, respectively. They crystallize in the noncentrosymmetric La3CuSiS7-type structure (hexagonal, space group P63, Z=2) in which the a-parameter is largely controlled by the RE component (a=10.0–10.3Å for the sulfides and 10.3–10.6Å for the selenides) whereas the c-parameter is essentially fixed by the choice of Ga and chalcogen atoms within tetrahedral units (c=6.1Å for the sulfides and 6.4Å for the selenides). They extend the series RE 3 MGaCh 7, previously known for divalent metal atoms (M=Mn–Ni), differing in that the Cu atoms in RE 3CuGaCh 7 occupy trigonal planar sites instead of octahedral sites. Among quaternary chalcogenides RE 3 MM′Ch 7, the combination of monovalent (M=Cu) and trivalent (M′=Ga) metals is unusual because it appears to violate the condition of charge balance satisfied by most La3CuSiS7-type compounds. The possibility of divalent Cu atoms was ruled out by bond valence sum analysis, magnetic measurements, and X-ray photoelectron spectroscopy. The electron deficiency in RE 3CuGaCh 7 is accommodated through S-based holes at the top of the valence band, as shown by band structure calculations on La3CuGaS7. An optical band gap of about 2.0eV was found for La3CuGaSe7.