Many of the fundamental questions regarding the solid-state chemistry of boron are still unsolved, more than 200 years after its discovery. Recently, theoretical work on the existence and stability of known and new modifications of the element combined with high-pressure and high-temperature experiments have revealed new aspects. A lot has also happened over the last few years in the field of reactions between boron and main group elements. Binary compounds such as B(6)O, MgB(2), LiB(1-x), Na(3)B(20), and CaB(6) have caused much excitement, but the electron-precise, colorless boride carbides Li(2)B(12)C(2), LiB(13)C(2), and MgB(12)C(2) as well as the graphite analogue BeB(2)C(2) also deserve special attention. Physical properties such as hardness, superconductivity, neutron scattering length, and thermoelectricity have also made boron-rich compounds attractive to materials research and for applications. The greatest challenges to boron chemistry, however, are still the synthesis of monophasic products in macroscopic quantities and in the form of single crystals, the unequivocal identification and determination of crystal structures, and a thorough understanding of their electronic situation. Linked polyhedra are the dominating structural elements of the boron-rich compounds of the main group elements. In many cases, their structures can be derived from those that have been assigned to modifications of the element. Again, even these require a critical revision and discussion.

Boron: elementary challenge for experimenters and theoreticians.

Chemical bonding models based on graph theory or tensor surface harmonic theory demonstrate the analogy between the aromaticity in two-dimensional planar polygonal hydrocarbons such as benzene and that in three-dimensional deltahedral borane anions of the type BnHn2- (6 < or = n < or = 12). Such models are supported both by diverse computational studies and experimental determinations of electron density distribution. Related methods can be used to study the chemical bonding in the boron polyhedra found in other structures including neutral binary boron hydrides, metallaboranes, various allotropes of elemental boron, and boron-rich solid-state metal borides.

Three-dimensional aromaticity in polyhedral boranes and related molecules.

Hydrides of period 2 and 3 elements are promising candidates for hydrogen storage but typically have heats of reaction that are too high to be of use for fuel cell vehicles. Recent experimental wor...

Identification of Destabilized Metal Hydrides for Hydrogen Storage Using First Principles Calculations

For decades, borides have been primarily studied as crystallographic oddities. With such a wide variety of structures (a quick survey of the Inorganic Crystal Structure Database counts 1253 entries for binary boron compounds!), it is surprising that the applications of borides have been quite limited despite a great deal of fundamental research. If anything, the rich crystal chemistry found in borides could well provide the right tool for almost any application. The interplay between metals and the boron results in even more varied material's properties, many of which can be tuned via chemistry. Thus, the aim of this review is to reintroduce to the scientific community the developments in boride crystal chemistry over the past 60 years. We tie structures to material properties, and furthermore, elaborate on convenient synthetic routes toward preparing borides.

Rediscovering the Crystal Chemistry of Borides.

https://iopscience.iop.org/article/10.1088/1468-6996/11/2/023001/pdf

Raman effect in icosahedral boron-rich solids

The crystal structure of a magnesium heptaboride, whose structural formula is Mg2B14, was determined from X-ray powder patterns according to a profile refinement technique and on the basis of isotypism with MgAlB14. The symmetry is orthorhombic and the lattice parameters are a = 5.970 A , b = 8.125 A and c = 10.480 A . The refinement is based on the space group Imam. The unit cell contains five non-equivalent boron and two nonequivalent magnesium atoms. The structure consists of chains of B12 icosahedra extending in the direction of the c axis. The chains are linked laterally via inter-icosahedral bonds or BB bonds involving nonicosahedral atoms, thus forming a three-dimensional boron network. The magnesium atoms occupy two types of holes in this network, corresponding to coordination numbers 12 and 16.

M. Barret

Papers

Crystal structure of magnesium heptaboride Mg2B14