Syntheses and Crystal Structures of the Lanthanum Titanium Oxyselenides La4Ti2O4Se5 and La6Ti3O5Se9

Abstract

The new compounds La4Ti2O4Se5 and La6Ti3O5Se9 have been obtained from stoichiometric reactions of La, Ti, TiO2, and Se in fused-silica tubes at 1223 K. Potassium halide fluxes were used to promote the crystallization. The compounds have been characterized by single-crystal X-ray diffraction methods. La4Ti2O4Se5 crystallizes in the orthorhombic space group Cmcm with eight formula units in a cell at 153 K of dimensions a=3.9695(4) Å, b=33.115(3) Å, c=18.6380(18) Å. The five independent La atoms in the structure are found in four different coordination polyhedra, namely tricapped trigonal prism, bicapped trigonal prism, square antiprism, and 7-octahedron, whereas the three independent Ti atoms have octahedral coordination. The crystal structure comprises infinite layers that stack along the b-axis. Two adjacent layers are linked by La–Se bonds. La4Ti2O4Se5 represents the first rare-earth titanium oxychalcogenide containing only Ti3+ cations. La6Ti3O5Se9 crystallizes in the monoclinic space group C2/m with four formula units in a cell at 153 K of dimensions a=43.999(6) Å, b=3.9454(6) Å, c=11.1093(16) Å, β=96.882(2)°. The six independent La atoms in the structure are found in three different coordination polyhedra, namely tricapped trigonal prism, bicapped trigonal prism, and 7-octahedron. The four independent Ti atoms have octahedral coordination. The crystal structure comprises infinite condensed layers of composition 2∞[La2Ti1O2Se3] that stack successively with extended slabs of composition 2∞[La4Ti2O3Se6] along the a-axis. La6Ti3O5Se9 is a mixed-valence compound in which two-thirds of the Ti cations are Ti3+ and one-third are Ti4+.