Synthesis and crystal structure of MgB12

Single crystals of MgB12 were synthesized from the elements in a Mg/Cu melt at 1600 °C. MgB12 crystallizes orthorhombic in space group Pnma   with a=16.632(3)Å, b=17.803(4)Å and c=10.396(2)Å. The crystal structure (Z=30Z=30, 5796 reflections, 510 variables, R1(F)=0.049R1(F)=0.049, wR2(I)=0.134wR2(I)=0.134) consists of a three dimensional net of B12 icosahedra and B21 units in a ratio 2:1. The B21 units are observed for the first time in a solid compound. Mg is on positions with partial occupation. The summation reveals the composition MgB12.35 or Mg0.97B12 , respectively. This is in good agreement with the value of MgB11.25 as expected by electronic reasons to stabilize the boron polyhedra B122- and B214-.

The new crystal structure of MgB12 is characterized by a Kagome net of B12 icosahedra and B21 units, which are observed for the first time. The Mg atoms are placed in voids of the framework.Figure optionsDownload as PowerPoint slide