Structure determination of La 3CuGeS 7 and La 3CuGeSe 7

Abstract

We have synthesized La 3CuGeS 7 and La 3CuGeSe 7 and characterized their structures from single crystal X-ray diffraction data. From lattice constant data alone, Guittard et al. [C.R. Acad. Sci. Paris 267 (1968) 767; Bull. Soc. Chim. Fr. 7 (1970) 2467] had assumed that both of these phases were members of a large family of hexagonal compounds with the general formula Ln 6MS 6(TX 4) 2 or Ln 6M 2′S 6(TX 4) 2, where Ln=lanthanide element, M=divalent element (Mg, Ni, Mn), M′=monovalent element (Cu, Ag), T=(Ge, Si, Al or Fe), and X=(S, Se); our data confirm this assumption. La 3CuGeS 7 and La 3CuGeSe 7 both contain isolated [Ge(S,Se)] 4− tetrahedra, trigonal planar Cu(S,Se) 3 units, and La counter-cations. Consistent with its assignment to a non-centrosymmetric space group ( P6 3), La 3CuGeSe 7 shows a weak second harmonic generation response when irradiated with 1064-nm laser light.