Synthesis and characterization of three new rare-earth titanium oxyselenides: Ln_3.67Ti₂O₃Se₆ (Ln = Ce, Nd, Sm)

Three new compounds, Ln_3.67Ti₂O₃Se₆ (Ln = Ce, Nd, Sm), have been prepared from the reaction in stoichiometric ratios of Ln, TiO₂, Ti, and Se in sealed fused-silica tubes at 950 °C. A KBr flux was used to promote the crystal growth. Single-crystal X-ray diffraction measurements show that these compounds are isostructural. They crystallize with four formula units in the monoclinic space group C2/m in a new structure type that contains four crystallographically independent Ln atoms with three different coordination geometries and three nonequivalent Ti atoms in octahedral geometry. The structure comprises infinite condensed slabs of composition (∞)/²[Ln₄Ti₂O₄Se₄Se(5/2)] alternating along the a axis with extended slabs of composition (∞)/²[Ln_3.33Ti₂O₂Se₃Se(5/2)]. To achieve charge balance, Ti³⁺ and Ti⁴⁺ cations must be present in equal numbers; these are randomly distributed over the Ti sites. Crystal data at -120 °C follow: Ce_3.67Ti₂O₃Se₆, a = 28.315(6) Å, b = 3.8719(8) Å, c = 11.271(2) Å, β = 90.45(3)°, R(F) = 0.0286; Nd_3.67Ti₂O₃Se₆, a = 28.121(6) Å, b = 3.8232(8) Å, c = 11.179(2) Å, β = 90.15(3)°, R(F) = 0.0322; Sm_3.67Ti₂O₃Se₆, a = 27.961(6) Å, b = 3.7827(8) Å, c = 11.128(2) Å, β = 90.26(3)°, R(F) = 0.0294. The magnetic susceptibilities of the Ln_3.67Ti₂O₃Se₆ (Ln = Ce, Nd, Sm) compounds are dominated by the trivalent rare-earth cations and display paramagnetic behavior down to 5 K.