New layered materials:: Syntheses, structures, and optical properties of K2TiCu2S4, Rb2TiCu2S4, Rb2TiAg2S4, Cs2TiAg2S4, and Cs2TiCu2Se4

The new compounds K2TiCu2S4, Rb2TiCu2S4, Rb2TiAg2S4, Cs2TiAg2S4, and Cs2TiCu2Se4 have been synthesized by the reactions of A(2)Q(3) (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 p4(2)/mcm of the tetragonal system in cells of dimensions a = 5.6046(4) Angstrom, c = 13.154(1) Angstrom for Rb2TiCu2S4, a = 6.024(1) Angstrom, c = 13.566(4) Angstrom for Cs2TiAg2S4, and a = 5.852(2) Angstrom, c = 14.234(5) Angstrom for Cs2TiCu2Se4 at 153 K. Their structure is closely related to that of Cs2ZrAg2Te4 and comprises (2)(infinity)[TiM(2)Q(4)(2-)] layers, which are separated by alkali metal atoms. The (2)(infinity)[TiM(2)Q(4)(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 A(2)TiM(2)Q(4) compound is assigned to a transition from an M d and Q p valence band (HOMO) to a Ti 3d conduction band.