SYNTHESIS AND CRYSTAL-STRUCTURE OF ORTHORHOMBIC NASN2CL5 - A NEW-TYPEOF AB(2)X(5) COMPOUND

The phase diagram of the quasi-binary system NaCl-SnCl2 has been deriv ed from DSC and X-ray investigations. In this system a dystectic NaSn2 Cl5 compound phase was found, which was obtained by grinding mixtures of NaCl and SnCl2 in a closed ball mill and then subsequent tempering at temperatures about 170 degrees C in evacuated sealed silica ampoule s. The single-crystal structure of NaSn2Cl5 was determined. It crystal lizes with a new orthorhombic-type arrangement [space group Pnnm; a = 818.09(5), b = 1203.1(1), c = 871.63(8) pm; Z = 4]. The structure was solved by Patterson methods with the MolEN program, and refined to a f inal R value of 0.019 for 717 unique reflections. The sodium ions are located in almost octahedral voids of Cl ions. Two kinds of tin ions m anifest themselves structurally by a great distortion of their coordin ation environment, three CI ions are situated at <272.77 pm from the t in ion, which is a reasonable Sn-CI band length, but the other five or six are repelled by the lone pair to distances of >306.75 pm. Differe nt from the structure of U3Se5, it is another distorted Rh5Ge3 structu re. In this paper the structural relationship between NH4Pb2Cl5 and Na Sn2Cl5 is elucidated systematically by means of the group theoretical representation of group-subgroup relations. (C) 1995 Academic Press, I nc.