Electronic Structure of Single-Crystalline Mg_xAl_(1-x)B_2 Probed by X-ray Diffraction Multipole Refinements and Polarization-Dependent X-ray Absorption Spectroscopy

摘要

X-ray diffraction multipole refinements of single-crystalline Mg_xAl_(1-x)B_2 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 p_z bonds and underline the importance of holes in the σ-bonded covalent sp~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 covalent B rings. For A1B_2 (x = 0), on the other hand, about 1.5 electrons per A1 atom are transferred to the B sheets while the residual 1.5 electrons remain at the A1 site which suggests significant covalent bonding between the A1 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 modified bonding nature and a completely different electron/hole distribution between the σ and π bonds. While the electrons are predominantly confined to the in-plane σ and π states in MgB_2, they are found in covalent bond clusters between Al and the adjacent B-B bonds and have a. more three-dimensional character for AlB_2.