paper

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

arXiv:1208.1684 · doi:10.7566/JPSJ.83.024714

Abstract

X-ray diffraction multipole refinements of single-crystalline Mg $_{x}$ Al $_{1 - x}$ B $_{2}$ and polarization-dependent near-edge x-ray absorption fine structure at the B 1 $s$ 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 $p_{z}$ bonds and underline the importance of holes in the $I ¨ $ -bonded covalent $s p^{2}$ states for the superconducting properties of the diborides. The data indicate that Mg is approximately divalent in MgB $_{2}$ and suggest predominantly ionic bonds between the Mg ions and the two-dimensional B rings. For AlB $_{2}$ ( $x = 0$ ), 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 equivalent electron counts on B sheets of MgB $_{2}$ and AlB $_{2}$ \@, yet with a completely different electron/hole distribution between the $I ¨ $ and $I ¨ $ bonds.

Electronic structure of single-crystalline Mgx​Al1−x​B2​ probed by x-ray diffraction multipole refinements and polarization-dependent x-ray absorption spectroscopy

Abstract

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