New layered materials: Syntheses, structures, and optical properties of k₂ticu₂s₄, rb₂ticu₂s₄, rb₂tiag₂s₄, and cs₂ticu₂se₄

The new compounds K₂TiCu₂S₄, Rb₂TiCu₂S₄, Rb₂TiAg₂S₄, Cs₂TiAg₂S₄, and Cs₂TiCu₂Se₄ have been synthesized by the reactions of A₂Q₃ (A = K, Rb, Cs; Q = S, Se) with Ti, M (M = Cu or Ag), and Q at 823 K. The compounds Rb₂TiCu₂S₄, Cs₂TiAg₂S₄, and Cs₂TiCu₂Se₄ are isostructural. They crystallize with two formula units in space group P4₂/mcm of the tetragonal system in cells of dimensions a = 5.6046(4) Å, c = 13.154(1) Å for Rb₂TiCu₂S₄, a =6.024(1) Å, c = 13.566(4) Å for Cs₂TiAg₂S₄, and a =5.852(2) Å, c =14.234(5) Å for Cs₂TiCu₂Se₄ at 153 K. Their structure is closely related to that of Cs₂ZrAg₂Te₄ and comprises _∞²[TiM₂Q₄^2-] layers, which are separated by alkali metal atoms. The _∞²[TiM₂Q₄^2-] layer is anti-fluorite-like with both Ti and M atoms tetrahedrally coordinated to Q atoms. Tetrahedral coordination of Ti⁴⁺ is rare in the solid state. On the basis of unit cell and space group determinations, the compounds K₂TiCu₂S₄ and Rb₂TiAg₂S₄ are isostructural with the above compounds. The band gaps of K₂TiCu₂S₄, Rb₂TiCu₂S₄, Rb₂TiAg₂S₄, and Cs₂TiAg₂S₄ are 2.04, 2.19, 2.33, and 2.44 eV, respectively, as derived from optical measurements. From band-structure calculations, the optical absorption for an A₂TiM₂Q₄ compound is.assigned to a transition from an M d and Q p valence band (HOMO) to a Ti 3d conduction band.