Synthesis and characterization of three new rare-earth titanium oxyselenides: Ln(3.67)Ti(2)O(3)Se(6) (Ln = Ce, Nd, Sm)

Three new compounds, Ln(3.67)Ti(2)O(3)Se(6) (Ln = Ce, Nd, Sm), have been pr epared from the reaction in stoichiometric ratios of Ln, TiO2, Ti, and Se i n sealed fused-silica tubes at 950 degrees C. A KBr flu. was used to promot e the crystal growth. Single-crystal X-ray diffraction measurements show th at these compounds are isostructural. They crystallize with four formula un its in the monoclinic space group C2/m in a hew structure type that contain s four crystallographically independent Ln atoms with three different coord ination geometries and three nonequivalent Ti atoms in octahedral geometry. The structure comprises infinite condensed slabs of composition (2)(infini ty)[Ln(4)Ti(2)O(4)Se(4)Se(5/2)] alternating along the a axis with extended slabs of composition (2)(infinity)[Ln(3.33)Ti(2)O(2)Se(3)Se(5/2)]. To achie ve charge balance, Ti3+ and Ti4+ cations must be present in equal numbers; these are randomly distributed over the Ti sites. Crystal data at -120 degr ees C follow: Ce3.67Ti2O3Se6, a = 28.315(6) Angstrom, b = 3.8719(8) Angstro m, c = 11.271(2) Angstrom, beta = 90.45(3)degrees, R(F) = 0.0286; Nd3.67Ti2 O3Se6, a = 28.121(6) Angstrom, b = 3.8232(8) Angstrom, c = 11.179(2) Angstr om, beta = 90.15(3)degrees, R(F)= 0.0322; Sm3.67Ti2O3Se6, a = 27.961(6) Ang strom, b = 3.7827(8) Angstrom, c = 11.128(2) Angstrom, beta = 90.26(3)degre es, R(F) = 0.0294. The magnetic susceptibilities of the Ln(3.67)Ti(2)O(3)Se (6) (Ln = Ce, Nd, Sm) compounds are dominated by the trivalent rare-earth c ations and display paramagnetic behavior down to 5 K.