Synthesis and Crystal Structure of the New Ternary Halide Series Ba₂MCl₇ (M = Gd−Yb, Y) Containing the Highly Efficient Up-Conversion Material Ba₂ErCl₇

Single crystals of the compounds Ba2MCl7 (M = Gd−Yb, Y) can be grown from stoichiometric melts of the binary chlorides using the Bridgman technique. The crystal structure of Ba2ErCl7 and Ba2GdCl7 was determined by direct and Fourier methods using single crystal X-ray diffraction data [Ba2ErCl7: monoclinic, space group P21/c, Z = 4, a = 6.794(2) Å, b = 15.525(2) Å, c = 10.496(2) Å, β = 90.54(2)°, 3216 reflections, R = 0.034; Ba2GdCl7: monoclinic, P21/c, Z = 4, a = 6.826(1) Å, b = 15.584(2) Å, c = 10.557(1) Å, β = 90.67(1)°, 3259 reflections, R = 0.028]. The compounds Ba2MCl7 (M = Gd−Yb, Y) are found to be isotypic, their lattice parameters were refined from X-ray powder data. A characteristic feature of the crystal structure is the isolated polyhedron of seven-coordinated M3+ ions, which may be described as a trigonal prism monocapped on a rectangular prismatic face. Ba2ErCl7 shows interesting near IR to near UV/vis up-conversion properties.