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X-ray diffraction multipole refinements of single-crystallineMg$_x$Al$_{1-x}$B$_2$ and polarization-dependent near-edge x-ray absorptionfine structure at the B 1$s$ edge reveal a strongly anisotropic electronicstructure. Comparing the data for superconducting compounds ($x= 0.8$, 1.0)with those for the non-superconductor ($x=0$) gives direct evidence for arearrangement of the hybridizations of the boron $p_z$ bonds and underline theimportance of holes in the $\sigma$-bonded covalent $sp^2$ states for thesuperconducting properties of the diborides. The data indicate that Mg isapproximately divalent in MgB$_2$ and suggest predominantly ionic bonds betweenthe Mg ions and the two-dimensional B rings. For AlB$_2$ ($x=0$), on the otherhand, about 1.5 electrons per Al atom are transferred to the B sheets while theresidual 1.5 electrons remain at the Al site which suggests significantcovalent bonding between the Al ions and the B sheets. This finding togetherwith the static electron deformation density points to almost equivalentelectron counts on B sheets of MgB$_2$ and AlB$_2$\@, yet with a completelydifferent electron/hole distribution between the $\sigma$ and $\pi$ bonds.

journal of the physical society of japan

Electronic structure of single-crystalline Mg$_x$Al$_{1-x}$B$_2$ probed  by x-ray diffraction multipole refinements and polarization-dependent x-ray  absorption spectroscopy