Seminar in Department of Physics: Unraveling mysteries of boron-rich solids through electron-density analysis

Title:  Unraveling mysteries of boron-rich solids through electron-density analysis

Speaker: Dr. Swastik Mondal, University of Bayreuth, Germany

Date: 01.03.13

Time: 11.10 a.m.

Venue: L3

Abstract:-

Several boron-rich solids including allotropes of boron show enigmatic physical properties like, low compressibility, super-hardness and superconductivity. Many of these physical properties are caused by the unusual bonding mechanisms of boron-rich solids. Ubiquitous building block of boron-rich solids is the twelve-atom B12 cluster with approximate icosahedral symmetry. Explanations of enigmatic properties of boron-rich solids thus require the understanding of bonding mechanism between B12 clusters and between B12 clusters and other atoms. The knowledge of electron-density distribution in these materials will provide insights into the nature of these interactions. Analysis of electron densities employing Bader’s quantum theory of atoms in molecules (QTAIM) [1] will characterize the nature of the chemical interactions between the atoms. Quantitative information on the properties of bonds can be obtained on the basis of the topological properties of electron densities and the associated Laplacians at bond critical points. Recently, on the basis of experimental electron-density studies [2, 3] of orthorhombic γ-boron and rhombohedral α-boron, we have provided explanations for unusual bonding mechanisms and charge transfers in these allotropes. Electron density studies were performed using multipole models refined against low-temperature, high-resolution single-crystal X-ray diffraction data. Final multipole models show excellent fits to diffraction data for both polymorphs (RF = 0.0116 for α-boron and RF = 0.0120 for γ-boron). From electron-density analysis, it was revealed that the spatial distribution of the electron density at the corners of the B12 cluster is strongly directional and pointing outward perpendicularly to the surface of the cluster. This exo-cluster orbital order is persistent in both α- and γ-boron, irrespective of the fact that they participate in completely different kinds of inter-cluster bonds. Analysis of other boron-rich materials also indicates the same orbital order, suggesting a unifying bonding mechanism in boron-rich solids.