Structure determination of La3CuGeS7 and La3CuGeSe7

We have synthesized La3CuGeS7 and La3CuGeSe7 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(6)MS(6)(TX4)(2) or Ln(6)M(2)'S,(TX4)(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. La3CuGeS7 and La3CuGeSe7 both contain isolated [Ge(S,Se)](4-) tetrahedra, trigonal planar Cu(S,Se), units, and La counter-cations. Consistent with its assignment to a non-centrosymmetric space group (P6(3)), La3CuGeSe7 shows a weak second harmonic generation response when irradiated with 1064-nm laser light. (C) 2002 Elsevier Science B.V. All rights reserved.