Electronic Structure of Single-Crystalline Mg_xAl_1−xB₂ Probed by X-ray Diffraction Multipole Refinements and Polarization-Dependent X-ray Absorption Spectroscopy

Abstract

X-ray diffraction multipole refinements of single-crystalline MgxAl1−xB2 and polarizationdependent 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 sp 2 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 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 equivalent electron counts on B sheets of MgB2 and AlB2, yet with a completely different electron/hole distribution between the σ and π bonds.

Experimental X-ray Charge-Density Studies─A Suitable Probe for Superconductivity? A Case Study on MgB2

“…66,67 Maps of the Laplacian of the electron density, ∇ 2 ρ(r), in planes showing nearest-neighbor boron−boron and boron−magnesium contacts are given in Figure 3 (topological characteristics and Laplacian maps for the other multipolar models in this paper are available in Table S11 and Figures S21 and S22 of the Supporting Information). In agreement with a description of MgB 2 as a pseudo-Zintl phase 57 and previous experimental 24,46,61,62 and theoretical results, 58−60 only the BCP at the midpoint of the B−B contact (#1 in Figure 3) shows a large This is also evident from the respective atomic charges of +1.6 e and −0.8 e [DFT: +1.6 e/−0.8 e] as a consequence of the pronounced charge transfer from the electropositive magnesium atoms toward the boron atoms which form a graphene-type network characterized by delocalized π electrons.…”

X-ray charge-density studies $-$ a suitable probe for superconductivity?

“…In agreement with a description of MgB 2 as a pseudo-Zintl phase 49 and previous experimental 18,40,53,54 and theoretical results, [50][51][52] only the bond-critical point (BCP) at the midpoint of the B-B contact (#1 in Fig. 3 BCP (1.0 e•Å −3 at 15 K compared to 0.9 e•Å −3 at RT) which results from a charge transfer from delocalized π to localized σ bonds in the hexagonal boron layers 7 (see the structural model in Fig.…”

“…This discrepancy may be due to the fact that the IAM approach does not account for charge transfer between ions, the presence of aspherical density deformations and deformations of the atomic core densities resulting from the latter two effects [45][46][47][48] which in turn affect the determination of precise sample compositions. 36 In case of the pseudo-Zintl phase MgB 2 , strong valence charge transfer from the magnesium cations to the covalently bonded anionic boron network 18,40,[49][50][51][52][53][54] may hamper the determination of magnesium occupation factors by standard XRD refinement techniques. 53 To study this possible pitfall in more detail we performed IAM structural refinements using neutral-atom scattering factors (i.e.…”

Coupling of lattice dynamics and configurational disorder in metal deficient Al1−δB2 from first-principles

“…. Purple open squares correspond to experimental reference values at room temperature 17,18,53. Colored (open) circles are the values obtained from our DFT calculations.…”