Quaternary rare-earth sulfides RE 3 M 0.5 M′S7 (M = Zn, Cd; M′ = Si, Ge)

Highlights: • Four series of sulfides RE₃M_0.5M′S₇ (M = Zn, Cd; M′ = Si, Ge) were prepared. • They adopt noncentrosymmetric hexagonal structures with half-occupied M sites. • La₃Cd_0.5SiS₇ and La₃Cd_0.5GeS₇ have optical band gaps of 2.3–2.7 eV. -- Abstract: The four series of quaternary sulfides RE₃Zn_0.5SiS₇ (RE = La–Nd, Sm, Gd–Er), RE₃Zn_0.5GeS₇ (RE = La–Nd, Sm, Gd–Er), RE₃Cd_0.5SiS₇ (RE = La–Nd, Sm, Gd–Ho), and RE₃Cd_0.5GeS₇ (RE = La–Nd, Sm, Gd, Tb) were prepared by reactions of the elements at 1050 °C. Analysis of their powder X-ray diffraction patterns indicates that they adopt the noncentrosymmetric hexagonal La₃Mn_0.5SiS₇-type structure (space group P6₃, Z = 2). The structures of two Cd-containing members were refined from single-crystal X-ray diffraction data (Gd₃Cd_0.5SiS₇, a = 9.9737(8) Å, c = 5.6287(4) Å; Gd₃Cd_0.5GeS₇, a = 9.9739(3) Å, c = 5.7296(4) Å). La₃Zn_0.5GeS₇ melts incongruently above 1300 °C. Ce₃Zn_0.5GeS₇ shows simple paramagnetic behaviour arising from non-interacting Ce³⁺ species. Optical band gaps of 2.7 eV for La₃Cd_0.5SiS₇, 2.3 eV for La₃Cd_0.5GeS₇, and 1.8 eV for Ce₃Zn_0.5GeS₇ were measured. Band structure calculations on La₃Cd_0.5SiS₇ and La₃Cd_0.5GeS₇ confirm the presence of band gaps whose magnitude is controlled by Si–S or Ge–S bonding interactions.