Prediction of Superhard BN_(2) with High Energy Density

Considering that pressure-induced formation of short,strong covalent bonds in light-element compounds can produce superhard materials,we employ structure searching and first-principles calculations to predict a new class of boron nitrides with a stoichiometry of BN_(2),which are stable relative to alpha-B and alpha-N_(2) at ambient pressure.At ambient pressure,the most stable phase has a layered structure(h-BN_(2)) containing hexagonal BN layers between which there are intercalated N_(2) molecules.At 25 GPa,a three-dimensional P4_(2)/mmc structure with single N-N bonds becomes the most stable.Dynamical,thermal,and mechanical stability calculations reveal that this structure can be recovered under ambient conditions.Its calculated stress-strain relations demonstrate an intrinsic superhard nature with an estimated Vickers hardness of ~43 GPa.This structure has a potentially high energy density of ~4.19 kJ/g.

Metal-Element-Incorporation Induced Superconducting Hydrogen Clathrate Structure at High Pressure

The recent observation of high critical temperature T_(c) in lanthanum and Yttrium hydrides confirms the key role of hydrogen cage(H-cage)in determining high superconductivity.Here,we present a new class of metastable H_(12) clathrate structures based on the icosahedral cI 24-Na that can be stabilized by incorporation of metal elements.Analysis shows that the charge transfer from metal atoms to H atoms contributes to forming the H_(12) clathrate.Nine dynamically stable structures are identified to exhibit superconductivity,and a maximum T_(c) of 28K is found in voids-doped Mo_(6)H_(24).Calculations reveal that the low T_(c) is attributed to the weak interaction between H atoms in each cage due to the long H–H distance.The current results provide a possible route to design H-cage containing superconductors.

Pressure-induced phase transition in transition metal trifluorides

As a fundamental thermodynamic variable, pressure can alter the bonding patterns and drive phase transitions leading to the creation of new high-pressure phases with exotic properties that are inaccessible at ambient pressure. Using the swarm intelligence structural prediction method, the phase transition of TiF_(3), from R-3c to the Pnma phase, was predicted at high pressure, accompanied by the destruction of TiF_6 octahedra and formation of TiF_8 square antiprismatic units. The Pnma phase of TiF_(3), formed using the laser-heated diamond-anvil-cell technique was confirmed via high-pressure x-ray diffraction experiments. Furthermore, the in situ electrical measurements indicate that the newly found Pnma phase has a semiconducting character, which is also consistent with the electronic band structure calculations. Finally, it was shown that this pressure-induced phase transition is a general phenomenon in ScF_(3), VF_(3), CrF_(3), and MnF_(3), offering valuable insights into the high-pressure phases of transition metal trifluorides.

Helium incorporation induced direct-gap silicides

The search of direct-gap Si-based semiconductors is of great interest due to the potential application in many technologically relevant fields.This work examines the incorporation of He as a possible route to form a direct band gap in Si.Structure predictions and first-principles calculations show that He and Si,at high pressure,form four dynamically stable phases of Si_(2)He(oP36-Si_(2)He,tP9-Si_(2)He,mC18-Si_(2)He,and mC12-Si_(2)He).All phases adopt host–guest structures consisting of a channel-like Si host framework filled with He guest atoms.The Si frameworks in oP36-Si2He,tP9-Si2He,and mC12-Si_(2)He could be retained to ambient pressure after removal of He,forming three pure Si allotropes.Among them,oP36-Si_(2)He and mC12-Si_(2)He exhibit direct band gaps of 1.24 and 1.34 eV,respectively,close to the optimal value(~1.3 eV)for solar cell applications.Analysis shows that mC12-Si_(2)He with an electric dipole transition allowed band gap possesses higher absorption capacity than cubic diamond Si,which makes it to be a promising candidate material for thin-film solar cell.