Synthesis and crystal structure of MgB{sub 12}

Single crystals of MgB{sub 12} were synthesized from the elements in a Mg/Cu melt at 1600deg. C. MgB{sub 12} crystallizes orthorhombic in space group Pnma with a=16.632(3)A, b=17.803(4)A and c=10.396(2)A. The crystal structure (Z=30, 5796 reflections, 510 variables, R{sub 1}(F)=0.049, wR{sub 2}(I)=0.134) consists of a three dimensional net of B{sub 12} icosahedra and B{sub 21} units in a ratio 2:1. The B{sub 21} units are observed for the first time in a solid compound. Mg is on positions with partial occupation. The summation reveals the composition MgB{sub 12.35} or Mg{sub 0.97}B{sub 12} , respectively. This is in good agreement with the value of MgB{sub 11.25} as expected by electronic reasons to stabilize the boron polyhedra B{sub 12}{sup 2-} and B{sub 21}{sup 4-}.