Destruction of noncentrosymmetry through chalcogenide salt inclusion.

A salt-inclusion samarium selenogermanate compound, NaSmGeSe4 x 0.25 Na2Se was isolated from a reaction of Na2Se, Sm, GeSe2, and Se. The new structure consists of isolated GeSe4 units and bicapped trigonal-prismatic SmSe8, which are linked together to form corrugated anionic layers. The topology of the layer is similar to the well-known layered compounds ALnQE4 (A = K, Rb, Cs; Ln = lanthanide ions; Q = Si, Ge; E = S, Se) with some subtle differences. A selenide anion and Na cations in the interlayer space form interesting structures where Se-centered trigonal-prismatic polyhedra of SeNa6 are edge-shared and pass through a twofold rotation axis. Hence, this compound crystallizes in a centrosymmetric space group in contrast to the noncentrosymmetric structures adopted by the ALnQE4 series of compounds. Raman and diffuse-reflectance spectra were also analyzed for the title compound.