Noncentrosymmetric rare-earth copper gallium chalcogenides RE3CuGaCh7 (RE=La–Nd; Ch=S, Se): An unexpected combination

Abstract The quaternary rare-earth chalcogenides RE 3 CuGaS 7 and RE 3 CuGaSe 7 ( RE =La–Nd) have been prepared by reactions of the elements at 1050 °C and 900 °C, respectively. They crystallize in the noncentrosymmetric La 3 CuSiS 7 -type structure (hexagonal, space group P 6 3 , Z =2) in which the a -parameter is largely controlled by the RE component ( a =10.0–10.3 A for the sulfides and 10.3–10.6 A for the selenides) whereas the c -parameter is essentially fixed by the choice of Ga and chalcogen atoms within tetrahedral units ( c =6.1 A for the sulfides and 6.4 A for the selenides). They extend the series RE 3 M Ga Ch 7 , previously known for divalent metal atoms ( M =Mn–Ni), differing in that the Cu atoms in RE 3 CuGa Ch 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 La 3 CuSiS 7 -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 3 CuGa Ch 7 is accommodated through S-based holes at the top of the valence band, as shown by band structure calculations on La 3 CuGaS 7 . An optical band gap of about 2.0 eV was found for La 3 CuGaSe 7 .