Unraveling Stable Vanadium Tetraboride and Triboride
by First-Principles Computations
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Abstract
Transition metal polyborides (e.g.,
tetraborides or triborides)
with intriguing boron configurations offer a unique combination of
excellent mechanical, superconducting, optical, and thermoelectric
properties. Unraveling the specific structures of polyborides is critical
to understanding their underlying physical and electronic properties.
Here, we perform first-principles calculations to focus on the predictions
of geometrical structures, relative stabilities, and mechanical and
electronic properties of vanadium tetraboride (VB<sub>4</sub>) and
triboride (VB<sub>3</sub>). VB<sub>4</sub> prefers to take <i>Cmcm</i> symmetry with a planar boron isosceles trapezoid, differing
from earlier graphene-like or rhomboid boron arrangements in tetraborides.
Upon compression, another new <i>Amm</i>2-type structure
is energetically favorable above 12.0 GPa and can be stabilized up
to 50 GPa. Both structures are qualified into incompressible and hard
materials, comparable to previous reported tetraborides. Predicted <i>C</i>2/<i>m</i>-type VB<sub>3</sub> possesses intriguing
puckered boron bilayers and graphene-like boron sheet and shows excellent
mechanical properties. Analysis of electronic structure, electron
density distributions, and Mulliken overlap population demonstrated
the significant contributions of special planar isosceles trapezoid
boron units to the great stability and excellent mechanical property
for VB<sub>4</sub>. Furthermore, the universal trend of structural
features and mechanical behaviors was analyzed for the available 3d
transition metal tetraborides