Electron density distribution in a single crystal of Mg_1-xB₂ [x=0.045(5)]

The electron density distribution in Mg_1-xB₂ has been obtained by the precise x-ray structure analysis on high-quality single crystals. A slight deficiency (x∼0.04) of Mg has been observed. The obtained sectional Fourier map in the B plane shows the triangle-shaped electron distribution extended along the B-B bonds, which indicates the typical sp² orbitals of the boron atom. Further, the difference Fourier maps clearly reveal the Bσ(sp²) covalent bonds within the B plane and Bπ(p_z) orbitals perpendicular to the B plane. Both σ and π bonds construct the two-dimensional honeycomb network of the B atoms along the (001) plane. In contrast, there is no covalency between the B and Mg atoms, indicating the ionic bonds between B and Mg layers.