New Information on the Na-Ti-Se Ternary System

Molten Salt Fluxes, Polychalcogenide, Polyselenide Na2Ti2Se9 was first discovered in the quaternary system Na/Ti/P/Se, but it can be prepared rationally by dissolving Ti in a Na2Sex flux at 375 °C. The silverish dark red crystals are air-and water-stable. A new method for synthesizing Na4TiSe4 is also reported. Na2Ti2Se9 crystallizes in space group P21, with a = 12.785(4), b = 14.848(4), c = 13.961(4) Å, ß = 94.556(4)°, R1 = 0.0592 and wR2 = 0.1336. The structure is similar to that of KsNaTi6Se27. It contains infinite one dimensional chains which run parallel to the c-axis and contain Ti4+ centers bonded to Se22- and Se2- species. The l/∞[Ti2(Se2)4Se]2- chains pack together in a hexagonal pattern. All of the titanium atoms are surrounded by a pentagonal bipyramid of selenium atoms. The selenium atoms bridge the Ti centers in two different ways. Na2Ti2Se9 is a semiconductor with a band gap, Eg, of ~ 1.02 eV. The Raman spectra are also reported.