Electronic Structure of Single-Crystalline MgxAl1−xB2 Probed by X-ray Diffraction Multipole Refinements and Polarization-Dependent X-ray Absorption Spectroscopy

X-ray diffraction multipole refinements of single-crystalline MgxAl1−xB2 and polarization-dependent near-edge x-ray absorption fine structure at the B 1s edge reveal a strongly anisotropic electronic structure. Comparing the data for superconducting compounds (x = 0.8, 1.0) with those for the non-superconductor (x = 0) gives direct evidence for a rearrangement of the hybridizations of the boron pz bonds and underline the importance of holes in the σ-bonded covalent sp2 states for the superconducting properties of the diborides. The data indicate that Mg is approximately divalent in MgB2 and suggest predominantly ionic bonds between the Mg ions and the two-dimensional covalent B rings. For AlB2 (x = 0), on the other hand, about 1.5 electrons per Al atom are transferred to the B sheets while the residual 1.5 electrons remain at the Al site which suggests significant covalent bonding between the Al ions and the B sheets. This finding together with the static electron deformation density points to almost equi...