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

The new compounds K2TiCu2S4, Rb2TiCu2S4, Rb2TiAg2S4, Cs2TiAg2S4, and Cs2TiC u2Se4 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 Rb2TiCu2S 4, Cs2TiAg2S4, and Cs2TiCu2Se4 are isostructural. They crystallize with two formula units in space group p4(2)/mcm of the tetragonal system in cells o f 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 = 5.85 2(2) Angstrom, c = 14.234(5) Angstrom for Cs2TiCu2Se4 at 153 K. Their struc ture 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 at oms tetrahedrally coordinated to Q atoms. Tetrahedral coordination of Ti4is rare in the solid state. On the basis of unit cell and space group deter minations, the compounds K2TiCu2S4 and Rb2TiAg2S4 are isostructural with th e 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 absorp tion 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.