SYNTHESIS AND CRYSTAL-STRUCTURE OF THE NEW TERNARY HALIDE SERIES BA(2)MCL(7) (M=GD-YB, Y) CONTAINING THE HIGHLY EFFICIENT UP-CONVERSION MATERIAL BA2ERCL7

Single crystals of the compounds Ba(2)MCl(7) (M = Gd-Yb Y) can he grow n from stoichiometric melts of the binary chlorides using the Bridgman technique. The crystal structure of Ba2ErCl7 and Ba2GdCl7 was determi ned by direct and Fourier methods using single crystal X-ray diffracti on data [Ba2ErCl7: monoclinic, space group P2(1)/c, Z = 4, a 6.794(2) Angstrom, b = 15.525(2) Angstrom, c = 10.496(2) Angstrom, beta = 90.54 (2)degrees, 3216 reflections, R = 0.034; Ba2GdCl7: monoclinic, P2(1)/c , Z = 4, a = 6.826(1) Angstrom, b = 15.584(2) Angstrom, c = 10.557(1) Angstrom, beta = 90.67(1)degrees, 3259 reflections, R = 0.028]. The co mpounds Ba(2)MCl(7); (M = Gd-Yb, Y) are found to be isotypic, their la ttice parameters were refined from S-ray powder data. A characteristic feature of the crystal structure is the isolated polyhedron of seven- coordinated M(3+) ions, which may be described as a trigonal prism mon ocapped on a rectangular prismatic face. Ba2ErCl7 shows interesting ne ar LR to near UV/vis up-conversion properties.