Kuo Bao

Lowest enthalpy polymorph of cold-compressed graphite phase

Based on an ab initio evolutionary algorithm, a novel carbon polymorph with an orthorhombic Cmcm symmetry is predicted, named as C carbon, which has the lowest enthalpy among the previously proposed cold-compressed graphite phases.

Mechanical and metallic properties of tantalum nitrides from first-principles calculations

The phase stability, mechanical properties and metallic properties of tantalum nitrides are extensively studied by means of first principles calculations. The relationship between nitrogen concentration and physical properties of tantalum nitrides has been systematically investigated. With the nitrogen concentration increasing, it is found that the feature of covalent bonding enhances and the directionality of the covalent bonding and hardness of tantalum nitrides reduce. While these make the ductility of tantalum nitrides improve with the nitrogen concentration increasing. The intensity of metallic properties of tantalum nitrides can be effectively adjusted by controlling the nitrogen concentration and pressure. When the tantalum: nitrogen ratio reaches Ta:N = 1:3, remarkable nitrogen–nitrogen bonds are found in TaN3. The hardness of TaN3 abnormally increases with reference to that of the preceding composition Ta3N5-II. The potential synthesis routes of tantalum nitrides are suggested.

Structural and dynamical properties of solid ammonia borane under high pressure

The structural and dynamical properties of solid ammonia borane were investigated by means of extensive density functional theory calculation up to 60 GPa. Molecular dynamics simulations suggest that the Cmc2(1) phase found by recent room-temperature x-ray diffraction experiments can be obtained from the Pmn2(1) structure at high pressure and low temperature. Two new high-pressure phases were found on further compression at room temperature. We also found that all three high-pressure phases have proton-ordered structures, and the separation of the NH(3) and BH(3) rotation observed in the simulations can be explained by their distinct rotational energy barriers. The role of dihydrogen bonds in the high-pressure phases is discussed.

Cubic gauche-CN: A superhard metallic compound predicted via first-principles calculations

In this paper, we suggest a novel potential superhard material, a new carbon nitride phase consisted of sp(3) hybridized bonds, possessing a cubic P2(1)3 symmetry (8 atoms/cell, labeled by cg-CN) which is similar to cubic gauche nitrogen (cg-N) by first-principles calculations. It is a metallic compound, while most of other superhard materials are insulators or semiconductors. The Vickers hardness of cg-CN is 82.56 GPa, and if we considered the negative effect of metallic component on hardness, it is 54.7 GPa, which is much harder than any other metallic materials. It is found that a three-dimensional C-N network is mainly responsible for the high hardness. Both elastic constant and phonon-dispersion calculations show that this structure remains mechanically and dynamically stable in the pressure ranges from 0 to 100 GPa. Furthermore, we compared our results with many other proposed structures of carbon nitride with 1:1 stoichiometry and found that only cg-CN is the most favorable stable crystal structure. Formation enthalpies calculations demonstrate that this material can be synthesizable at high pressure (12.7-36.4 GPa).

Nitrogen concentration driving the hardness of rhenium nitrides.

The structures and properties of rhenium nitrides are studied with density function based first principle method. New candidate ground states or high-pressure phases at Re:N ratios of 3:2, 1:3, and 1:4 are identified via a series of evolutionary structure searches. We find that the 3D polyhedral stacking with strong covalent N-N and Re-N bonding could stabilize Re nitrides to form nitrogen rich phases, meanwhile, remarkably improve the mechanical performance than that of sub-nitrides, as Re3N, Re2N, and Re3N2. By evaluating the trends of the crystal configuration, electronic structure, elastic properties, and hardness as a function of the N concentration, we proves that the N content is the key factor affecting the metallicity and hardness of Re nitrides.

High-temperature superconductivity in compressed solid silane.

Crystal structures of silane have been extensively investigated using ab initio evolutionary simulation methods at high pressures. Two metallic structures with P21/c and C2/m symmetries are found stable above 383 GPa. The superconductivities of metallic phases are fully explored under BCS theory, including the reported C2/c one. Perturbative linear-response calculations for C2/m silane at 610 GPa reveal a high superconducting critical temperature that beyond the order of 102 K.

Pressure induced phase transition in MH2 (M = V, Nb)

High-pressure structures of MH2 (M = V, Nb) are explored through ab initio evolutionary methodology. As the same main group metal hydrides, VH2 and NbH2 adopt the same tetragonal structure with space group Fm-3m at low pressures. However, at high pressures VH2 and NbH2 possess Pnma and P6₃mc phases differently. The two phase transitions are both the first order phase transition identified by volume collapses. Our calculations suggest that two high-pressure structures have both dynamical and mechanical stability up to 100 GPa. Pnma VH2 and P6₃mc NbH2 are metallic phases demonstrated by the band structure and density of states. However, their superconducting temperatures are only several Kelvins.

Manganese borides synthesized at high pressure and high temperature

We synthesized various kinds of manganese borides by liquid-solid reaction from manganese and amorphous boron powders at high pressure and high temperature. The mixed powders with various atomic ratio of boron to manganese, B/Mn = 0.6, 1.2, 2, 3, 4, 8, were treated at temperature 1100–1350 °C and pressure 4.8-5.5 GPa, for 10–285 min. Typical manganese borides such as Mn4B, Mn2B, MnB, Mn3B4, MnB2, MnB4, and MnBx were synthesized. MnB2 plays an important role in the reaction, which is the middle product and it changes to Mn3B4 and boron with increasing holding time at a mixed atomic ratio of B/Mn = 2 and MnB2 reacted with boron to MnB4 with increasing holding time at a mixed atomic ratio of B/Mn = 8. The preferred orientation of Mn3B4 was also obtained at a mixed atomic ratio B/Mn = 2 and whose growth is highly oriented. The crystalline phases MnB2, Mn3B4, MnB, and Mn2B were prepared with large excess boron content.

Pressure-Stabilized Superconductive Ionic Tantalum Hydrides

High-pressure structures of tantalum hydrides were investigated over a wide pressure range of 0-300 GPa by utilizing evolutionary structure searches. TaH and TaH2 were found to be thermodynamically stable over this entire pressure range, whereas TaH3, TaH4, and TaH6 become thermodynamically stable at pressures greater than 50 GPa. The dense Pnma (TaH2), R3̅m (TaH4), and Fdd2 (TaH6) compounds possess metallic character with a strong ionic feature. For the highly hydrogen-rich phase of Fdd2 (TaH6), a calculation of electron-phonon coupling reveals the potential high-Tc superconductivity with an estimated value of 124.2-135.8 K.

Crossover from metal to insulator in dense lithium-rich compound CLi4.

Significance The binary compound researched here enriches the studies of antimetallization just like in the pure elemental system, and the fundamental nature of matter in the subject has been expanded. During metallizing or antimetallizing in metallic states, the Fermi surface filling parameter is found to be a valuable parameter to quantify the evolution of the free electrons. At room environment, all materials can be classified as insulators or metals or in-between semiconductors, by judging whether they are capable of conducting the flow of electrons. One can expect an insulator to convert into a metal and to remain in this state upon further compression, i.e., pressure-induced metallization. Some exceptions were reported recently in elementary metals such as all of the alkali metals and heavy alkaline earth metals (Ca, Sr, and Ba). Here we show that a compound of CLi4 becomes progressively less conductive and eventually insulating upon compression based on ab initio density-functional theory calculations. An unusual path with pressure is found for the phase transition from metal to semimetal, to semiconductor, and eventually to insulator. The Fermi surface filling parameter is used to describe such an antimetallization process.