Zn_5Ir_7B_3, Zn_5Rh_7B_3, and Zn_(7 + x)Rh_(9 - x)B_3 (x approx= 0.4) -New Ternary Zinc Platinum Metal Borides

摘要

The new ternary zinc borides Znic borides Zn_5Ir_7B_3, Zn_5Rh_7B_3, and Zn_(7 + x)Rh_(9-x)B_3 (x approx= 0.4) were prepared by reaction of the elemental components at temperatures in the range 1200 to 1230 deg C. They crystallize orthorhombically in the space group Pmma with Z = 2. Zn_5Ir_7B_3 (a =116.1(2) pm, b = 284.96(4) pm, c = 1178.1(2) pm; R = 0.042, 1414 reflections, 47 parameters) and Zn_5RH_7b_3 (a = 1101.6(2) pm, b = 283.94(3) pm, c = 1166.6(4) pm, R = 0.033, 787 reflections, 47 parameters) are isotypic. Along the short axis planar nets of platinum metal atoms at y = 0 alternate with layers containing the boron and zinc atoms at Z = 1/2. By the stacking of the platinum metal nets columns of trigonal prisms centered by boron atoms, columns of pentagonal prisms containing zinc atoms and channels with horse shoe shaped cross sections, all running along the b-axis are formed. The latter are filled by an aggregation of zinc atoms consisting of four parallel rows. In the structure of Zn_(7 + x)rh_(9 - x)b_3 (a = 1117.1(3) pm, b = 285.38(8) pm, c = 1484.8(5) pm; R = 0.026, 975 reflections, 59 parameters) one of the sitesets is occupied by Rh and Zn atoms approximately in the ratio 6:4. The structure contains the same building elements as those found in Zn_5Rh_7B_3 and in addition Rh prisms with elongated hexagon cross sections accommodating pairs of zinc atoms. These prisms are connected by common faces to form layers perpendicular to the c axis.