USD 546.27

A. R. Oganov (New York, USA, Moscow, Russia)
V. L. Solozhenko (Villetaneuse, France)
C. Gatti (Milano, Italy)
O. O. Kurakevych, Y. Le Godec (Paris, France)

The high-pressure phase of boron, g-B₂₈: disputes and conclusions of 5 years after discovery

g-B₂₈ is a recently established high-pressure phase of boron. Its structure consists of icosahedral B₁₂ clusters and B₂ dumbbells in a NaCl-type arrangement (B₂)^d+(B₁₂)^d– and displays a significant charge transfer d » 0.5–0.6. The discovery of this phase proved to be essential for the understanding and construction of the phase diagram of boron. It was first experimentally obtained as a pure boron allotrope in early 2004 and its structure was discovered in 2006. This paper reviews recent results and contentious issues related to the equation of state, hardness, putative isostructural phase transformation at ~ 40 GPa, and debates on the nature of chemical bonding in this phase. Our analysis confirms that (a) calculations based on density functional theory give an accurate description of its equation of state, (b) the reported isostructural phase transformation in g-B₂₈ is an artifact, (c) the best estimate of hardness of this phase is 50 GPa, (d) chemical bonding in this phase has a significant degree of ionicity. Apart from presenting an overview of the previous results within a consistent view grounded in experiment, thermodynamics and quantum mechanics, we present new results on Bader charges in g-B₂₈ using different levels of quantum-mechanical theory (GGA, exact exchange, and HSE06 hybrid functional), and show that the earlier conclusion about a significant degree of a partial ionicity in this phase is very robust. An additional insight into the nature of the partial ionicity is obtained from a number of boron structures theoretically constructed in this work.

Keywords: boron, g-B₂₈, high-pressure phase.

 

 USD 546.27

P. Macchi (Bern, Switzerland)

On the nature of chemical bonding in g-boron

The chemical bonding in g-boron has been discussed, based on the known crystal structures of the new recently discovered phase. The unexpected polarity of some B–B bonds and a sensible electron counting scheme are presented. The relationship between the chemical bonding and the material hardness is also discussed.

Keywords: boron, chemical bond, bond polarity.

  

USD 546.27

Y. Le Godec (Paris, France)

Comparative review of theoretical and experimental equations of state of orthorhombic boron g-B₂₈

Various theoretical and experimental 300 K equations of state of g-B₂₈, a new ultrahard high pressure orthorhombic phase of boron, have been reviewed. Throughout analyses and critical comparisons, it has been shown that DFT-GGA is capable of describing the EOS of g-B₂₈ very accurately and that the suggestion, in the literature, of an isostructural phase transformation in g-B₂₈ at about 40 GPa is inconclusive. 

Keywords: boron, g-B₂₈ boron phase, equation of state, isostructural phase transformation, ab initio calculations.

  

USD 546.27

P. Rulis, L. Wang, B. Walker, W.-Y. Ching (Kansas City, USA)

Spectral analysis of the electronic structure of g-B₂₈

Elemental boron and boron-rich compounds have been the subjects of intense theoretical and practical interest and seem to be surrounded by an almost unceasing controversy with respect to their fundamental atomic and electronic structures. The sensitivity of boron to the presence of impurities leads to exceptional difficulty in preparing material samples of a specific purity or stoichiometry. Hence, the detailed effort to map out and understand the atomic and electronic structures of these materials, which has been painstakingly performed by numerous researchers over many decades, has often been fraught with missteps. However, when a theoretical or experimental method finally has proven to be successful in dealing with boron and its compounds it is often heralded as a triumph of science and is thus linked with the development of a deeper and more fundamental understanding of the physics of atomic interaction. This focused review article provides an analysis of the current state of understanding the atomic and electronic structures of elemental boron and boron-rich compounds with a strong bent towards the parts of the story related to the g-B₂₈ modification of pure boron. Also, while many techniques have been instrumental, the focus here will be on the results obtained from experimental and theoreti­cal/ computational methods of core level spectroscopy.

Keywords: boron, g-B₂₈, the atomic and electronic structures, impurity, spectroscopy.

  

USD 546.27

X.-F. Zhou (Tianjin, China)
Y. Tian (Qinhuangdao, China)
H.-T. Wang (Tianjin, Nanjing, China)

Large shear strength enhancement of gamma-boron by normal compression

The effects of normal compression on the mechanical properties of boron phases have been investigated using the first principles. The results have shown that normal compression produces entirely different effects on gamma-boron and alpha-boron shear strengths. The three-center bonds connecting the B₂ dumbbell pair with the B₁₂ icosahedra in gamma-boron are greatly strengthened compared with the peculiar bonds in the alpha-boron, resulting in a 32 % enhancement in gamma-boron shear strength. The current study provides an atomic understanding of the discrepancy between the theoretical and experimental indentation strength of gamma-boron.

Keywords: gamma-boron, alpha-boron, shear strength, Vickers hardness, first-principle calculations.

 
 

USD 546.27

S. Veprek, R. F. Zhang (Garching, Germany)
A. S. Argon (Cambridge, USA)

Mechanical properties and hardness of boron and boron-rich solids

A brief review of the recent studies of the crystal and electronic structure and its stability under large strain-shear and tensile deformation of selected boron compounds, which have been predicted to be superhard on the basis of their high elastic moduli, is presented. It is shown that in many cases, the materials undergo electronic instability and transformation to softer phases with a lower shear resistance than the original equilibrium structure. Therefore, high values of elastic moduli (“low compressibility”) do not guarantee high hardness. These results also challenge the recent “models of theoretical hardness of an ideal crystal”, which are based on the equilibrium electronic properties. It is shown that appropriately nanostructured materials open the way to the design of superhard materials.

Keywords: boron compounds, hardness, mechanical properties, first-principles calculations.

  

USD 546.27

O. O. Kurakevych (Paris, France)
V. L. Solozhenko (Villetaneuse, France)

Experimental study and critical review of structural, thermodynamic and mechanical properties of superhard refractory boron suboxide B₆O

In the present study the analysis of available data on structural, thermodynamic and mechanical properties of B₆O has been performed. Although the compound is known for half a century and has been extensively studied, many properties of this boron-rich solid remain unknown or doubtful. A semi-empirical analysis of our experimental and literature data has allowed us to choose the best values of main thermodynamic and mechanical characteristics among previously reported data, to predict the thermoelastic equation of state of B₆O, and dependence of its hardness on non-stoichiometry and temperature.

Keywords: superhard materials, boron suboxide, thermodynamics, crystal structure, mechanical properties.