CHARACTERIZATION OF CE-3(SIS4)(2)I, A COMPOUND WITH A NEW STRUCTURE TYPE

The first cerium iodothiosilicate, Ce-3(SiS4)(2)I, has been synthesize d from the reaction of cerium sulfide with silicon, iodine and sulfur at high temperature. This compound crystallizes in the monoclinic symm etry (space group C2/c and Z=4)with the powder refined cell parameters : a = 15.9634(5), b = 7.8502(2), c = 10.8664(3) Angstrom, beta = 97.93 1(2)degrees. The crystal structure was refined to R(%)= 2.17 and R-W(% )= 2.60 from single crystal X-ray diffraction data. Ce(SiS4)(2)I prese nts tunnels in which are located the iodide anions surrounded by three ceriums, in [ICe3] isosceles triangular entities. The tunnels are con stituted of a three dimensional network made of (CeIS8) polyhedra (tri gonal prisms of sulfur tricapped by two sulfur and one iodine atom) li nked to (SIS,) tetrahedra. Magnetic susceptibility measurements and UV -VIS diffuse reflectance spectroscopy are consistent with the occurren ce of Ce-III ions, whereas band structure calculations indicate that t he phase is a semiconductor. The charge balance in the material can be written as : (Ce3Si2I8-I -II)-Si-III-I-IV.