Deformation Induced Solid–Solid Phase Transitions in Gamma Boron

We predict three new polymorphs of
boron by applying density functional
theory (PBE flavor) to large shear deformations starting from the
recently discovered γ-B28 boron phase (stable above
9 GPa and 1000 K). We find that continuous deformation along the (100)/⟨001⟩
slip system leads to two new phases, named here as γ-B12–(B2)6 and γ-B12–(B···B)6. We show that these γ-B12–(B2)6 and γ-B12–(B···B)6 phases can also be obtained from uniaxial tensile and compressive
deformations of the γ-B28 phase along the ⟨101⟩
direction, respectively. However, the reverse compressive loading
on the newly formed γ-B12–(B2)6 phase transforms it to itself, not the γ-B28 phase, because of the transferability of the three-center two-electron
bond under deformation. This makes the new phase γ-B12–(B2)6 a special type of superelastic
material. In addition, application of reverse tensile deformation
on the newly formed γ-B12–(B···B)6 phase, transforms it to a third new phase, named α-B12–BB, that is metallic, suggesting increased ductility
that might make α-B12–BB important for applications
in electronic devices. We compared the structural character, mechanical
properties, and electronic properties of these new phases to each
other and to other phases of boron. We show that the three new phases
are dynamically stable at zero pressure. These results show how modifying
the connections between boron icosahedra using one to two atom chains
can lead to dramatically different mechanical and electronic properties.