Ultrasonics at Loughborough University

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+.