A new quaternary alkali metal, rare earth thiophosphate was synthesized by the ceramic method and characterized by single crystal X-ray diffraction: K(2)SmP(2)S(7) Crystallizes in the monoclinic space group C2 (no. 5) with the unit cell parameters a = 22.746(5) angstrom, b = 6.7362(13) angstrom, c = 8.9004(18) angstrom, = 99.68(3)degrees, V = 1344.3(5) angstrom(3) and Z = 2. The phase K(2)SmP(2)S(7) can be better described as K(4)Sm(2)[PS(4)](2)[P(2)S(6)] because it contains in the crystal structure both discrete anions [PS(4)](3-) and [P(2)S(6)](4-). The crystal structure consists of infinite chains of (1)(infinity)[SmPS(4)] that are linked together through the ethane-like [P(2)S(6)](4-) anions to form two-dimensional layers. The packing of K(2)SmP(2)S(7) is formed by layers of (2)(infinity)[Sm(2)[PS(4)](2)[P(2)S(6)]](4-) separated by K(+) cations. K(2)SmP(2)S(7) was characterized by Raman and Fourier transform infrared (FTIR) spectroscopy and SEM-EDX microprobe analyses. The optical band gap of K(2)SmP(2)S(7) was determined by UV-vis diffuse reflectance measurements to be 2.59 eV. (C) 2007 Elsevier Ltd. All rights reserved.