A new quaternary alkali metal, rare earth thiophosphate was synthesized by the ceramic method and characterized by single crystal X-ray diffraction: K2SmP2S7 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 K2SmP2S7 can be better described as K4Sm2[PS4](2)[P2S6] because it contains in the crystal structure both discrete anions [PS4](3-) and [P2S6](4-). The crystal structure consists of infinite chains of (1)(infinity)[SmPS4] that are linked together through the ethane-like [P2S6](4-) anions to form two-dimensional layers. The packing of K2SmP2S7 is formed by layers of (2)(infinity)[Sm-2[PS4](2)[P2S6]](4-) separated by K+ cations. K2SmP2S7 was characterized by Raman and Fourier transform infrared (FTIR) spectroscopy and SEM-EDX microprobe analyses. The optical band gap of K2SmP2S7 was determined by UV-vis diffuse reflectance measurements to be 2.59 eV. (C) 2007 Elsevier Ltd. All rights reserved.