First principles-based electronic structure calculations of superhard iron tetraboride (FeB 4 ) under high pressure have been undertaken in this study. Starting with a "conventional" superconducting phase of this material under high pressure leads to an unexpected phase transition toward a semiconducting one. This transition occurred at 53.7 GPa, and this pressure acts as a demarcation between two distinct crystal symmetries, metallic orthorhombic and semiconducting tetragonal phases, with Pnnm and I4 1 /acd space groups, respectively. In this work, the electron-phonon coupling-derived superconducting T c has been determined up to 60 GPa and along with optical band gap variation with increasing pressure up to 300 GPa. The dynamic stability has been confirmed by phonon dispersion calculations throughout this study.metal-semiconductor phase transition | superhard material | first principle study | high pressure | superconductivity T he shorter interatomic distances of metal under external pressure consequently increase the valence and conduction band widths, which leads to the enhancement of free electronlike behavior. The development of creating immensely substantial pressure at laboratories enables us to observe the core electrons overlapping under enormous compression and dramatically influences the electronic properties of normally free electron metals such as Li and Na (1-3). The metalto-insulating phase transformation has been contrived both experimentally and theoretically for both the normal metals while exerting pressure on them. This observation propelled us to investigate the electronic and structural phase transformation of the experimentally synthesized superhard material iron tetraboride (FeB 4 ) under high pressure (4-8). The intriguing factor of choosing FeB 4 is that the material was proposed as a "conventional" Fe-based superconductor, in contradiction to the discovery of an "unconventional" Febased superconductor because of its large electron-phonon coupling. Here we report the exotic phase transition of FeB 4 from metal to semiconductor at 53.7 GPa, even though we started with the metallic orthorhombic phase Pnnm of FeB 4 , which shows the superconducting temperature T c up to 60 GPa. The new phase after 53.7 GPa has I4 1 =acd space group symmetry with a finite fundamental band gap, which increases along with pressure monotonically. All of the considered structures have been tested to have a thermodynamic stability from phonon dispersion calculations. The reason behind the phenomena could be the overlap of atomic cores at higher pressure ranges, which increases the hybridization of valence electrons and their repulsive interactions with core electrons. The immediate technological outcome of this scenario of metal-to-semiconducting phase transition could be to search for a transparent state of a material that is a metal under ambient conditions. This drastic change of electronic and structural properties can be observed in other materials as well, and hence this can open a field of studying them...

Employing a systematic first-principles investigation with crystal structure searching based on an evolutionary algorithm, we have uncovered the novel phase (P4 2 /nmc) of OsB 4 with a novel superhardness and semiconducting state. In this investigation, metal-to-semiconductor phase transition is predicted at only a few gigapascals above ambient pressure, i.e., 11 GPa. As a result, the P4 2 /nmc phase should potentially become a metastable phase at ambient pressure. The Vickers (polycrystalline) hardness and the band gap of the semiconducting phase are calculated to be 60 GPa and 2.90 eV, respectively. These findings indicate that the P4 2 /nmc phase might be a promising superhard-semiconducting material which could be used in cutting and drilling tools, material coating, and other advanced optical technologies. Moreover, under further compression up to 300 GPa, the semiconducting phase transforms into a metallic P6 3 /mmc phase at 134 GPa, and then another predicted metallic phase with a Cmca symmetry emerges beyond 270 GPa. Both dynamic and elastic stabilities are fully investigated to ensure the existence of the predicted phases.

This work reports the ab initio study of the crystal energetics, electronic properties, mechanical properties, and the impacts of strain on electronic properties of Mo2CT2 (T = O, F, and Cl).

The complex and unusual high-pressure phase transition of III-A (i.e. Al, Ga, and In) metals have been investigated in the last several decades because of their interesting periodic table position between the elements having metallic and covalent bonding. Our present first principles-based electronic structure calculations and experimental investigation have revealed the unusual distortion in face-centered cubic (f.c.c.) phase of the heavy element thallium (Tl) induced by the high pressure. We have predicted body-centered tetragonal (b.c.t) phase at 83 GPa using an evolutionary algorithm coupled with ab initio calculations, and this prediction has been confirmed with a slightly distorted parameter ( ffiffiffi 2 p × a − c)/c lowered by 1% using an angledispersive X-ray diffraction technique. The density functional theory (DFT)-based calculations suggest that s-p mixing states and the valence-core overlapping of 6s and 5d states play the most important roles for the phase transitions along the pathway h.c.p → f.c.c. → b.c.t.thallium | high pressure | phase transition | distorted face-centered cubic | first principle study