New information on the Na-Ti-Se ternary system

Na₂Ti₂Se₉ was first discovered in the quaternary system Na/Ti/P/Se, but it can be prepared rationally by dissolving Ti in a Na₂Se_x 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, β = 94.556(4)°, R1 = 0.0592 and wR2 = 0.1336. The structure is similar to that of K₅NaTi₆Se₂₇. It contains infinite one dimensional chains which run parallel to the c-axis and contain Ti⁴⁺ centers bonded to Se₂^2- and Se^2- species. The 1/∞[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, E_g, of ≃ 1.02 eV. The Raman spectra are also reported.