19 research outputs found
The high-pressure phase of boron, {\gamma}-B28: disputes and conclusions of 5 years after discovery
{\gamma}-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){\delta}+(B12){\delta}- and displays a significant charge
transfer {\delta}~0.5- 0.6. The discovery of this phase proved essential for
the understanding and construction of the phase diagram of boron. {\gamma}-B28
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 in
particular deals with the 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
{\gamma}-B28 is an artifact rather than a fact, (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 previous
results within a consistent view grounded in experiment, thermodynamics and
quantum mechanics, we present new results on Bader charges in {\gamma}-B28
using different levels of quantum-mechanical theory (GGA, exact exchange, and
HSE06 hybrid functional), and show that the earlier conclusion about
significant degree of partial ionicity in this phase is very robust
PRECISION X-RAY-DIFFRACTION STUDY OF THE GARNETS NA3SC2V3O12 AND NA0.90CA2.38MN1.72V3O12
The distribution of electron density and electrostatic potential in the compounds Na3Sc2V3O12 (ScVG) and Na0.90Ca2.38Mn1.72V3O12 (MnVG) with garnet structure was studied by precision x-ray diffraction. Refinement of the structure using reflections chosen by a special method from the outer region of reciprocal space led to R = 0.0085, R(w) = 0.0071, S = 1.006 (ScVG) and R = 0.0058, R(w) = 0.0049, S = 1.079 (MnVG). Maps of deformed electron density and deformed electrostatic potential were constructed and their interpretation given in terms of MO. Features of the electronic structure in the crystals studied have a confidence level not lower than 95%
PRECISION X-RAY-DIFFRACTION STUDY OF THE GARNETS NA3SC2V3O12 AND NA0.90CA2.38MN1.72V3O12
The distribution of electron density and electrostatic potential in the compounds Na3Sc2V3O12 (ScVG) and Na0.90Ca2.38Mn1.72V3O12 (MnVG) with garnet structure was studied by precision x-ray diffraction. Refinement of the structure using reflections chosen by a special method from the outer region of reciprocal space led to R = 0.0085, R(w) = 0.0071, S = 1.006 (ScVG) and R = 0.0058, R(w) = 0.0049, S = 1.079 (MnVG). Maps of deformed electron density and deformed electrostatic potential were constructed and their interpretation given in terms of MO. Features of the electronic structure in the crystals studied have a confidence level not lower than 95%