Na(3)MgB(37)Si(9): an icosa-hedral B(12) cluster framework containing {Si(8)} units.

Single crystals of a novel sodium-magnesium boride silicide, Na(3)MgB(37)Si(9) [a = 10.1630†(3)†à , c = 16.5742†(6)†à , space group R m (No. 166)], were synthesized by heating a mixture of Na, Si and crystalline B with B(2)O(3) flux in Mg vapor at 1373†K. The Mg atoms in the title compound are located at an inter-stitial site of the Dy(2.1)B(37)Si(9)-type structure with an occupancy of 0.5. The (001) layers of B(12) icosa-hedra stack along the c-axis direction with shifting in the [-a/3, b/3, c/3] direction. A three-dimensional framework structure of the layers is formed via B-Si bonds and {Si(8)} units of [Si](3)-Si-Si-[Si](3).