Xilian Jin

Superconducting high-pressure phases of disilane

High-pressure structures of disilane (Si2H6) are investigated extensively by means of first-principles density functional theory and a random structure-searching method. Three metallic structures with P-1, Pm-3m, and C2/c symmetries are found, which are more stable than those of XY3-type candidates under high pressure. Enthalpy calculations suggest a remarkably wide decomposition (Si and H2) pressure range below 135 GPa, above which three metallic structures are stable. Perturbative linear-response calculations for Pm-3m disilane at 275 GPa show a large electron-phonon coupling parameter λ of 1.397 and the resulting superconducting critical temperature beyond the order of 102 K.

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.

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.

A novel stable hydrogen-rich SnH8 under high pressure

A first-principles calculation is applied to perform a comprehensive study of the Sn–H system. Besides the common tetravalent hydride, a novel SnH8 crystal with the space group Im2 is reported with the most dominant enthalpy from structure searching techniques. All the H atoms of SnH8 are in the form of H2 or H3 units with electrons localized around them, showing covalent bond character. The rich and multiple Fermi surface distribution displays a metallic feature. Further electron–phonon coupling calculations reveal the high Tc of 63–72 K at 250 GPa.

High-pressure close-packed structure of boron

Based on ab initio evolutionary algorithms, a high-pressure close-packed phase of boron with hexagonal P63/mcm symmetry is predicted, named as B10, which is stable over α-Ga phase above 375 GPa to at least 500 GPa. High pressure makes the typical B12 icosahedron collapse to form an incompressible linear atomic chain arrangement together with an isosceles triangle arrangement. The electron localization function calculations confirm that the B10 has strong covalency in this special atomic arranged structure. The vibration of the three atoms isosceles triangle in the framework of linear atomic chains induces an unusual superconductivity in B10. Electron–phonon calculations indicate that electron–phonon coupling parameter λ is 0.82 and the superconducting critical temperature is 44 K at 400 GPa.

How to get superhard MnB2: a first-principles study

In 2009, a super-hard MnB2 with ReB2-type structure was predicted as being in the ground state. However, it has not been synthesized successfully in about two years either by high temperature and high pressure (HTHP) method or by the arc-melting method. To obtain the accurate synthesis conditions, the P–T phase boundary between AlB2-type and ReB2-type MnB2 has been studied by first-principles lattice dynamics calculations within quasi-harmonic approximation (QHA). Our results show that the ReB2-type MnB2 can be synthesized only below 1020 K at ambient pressure. Pressure effect makes their transition temperature decrease. If the pressure is higher than 38 GPa, only AlB2-type MnB2 can be obtained. The synthesis temperatures of previous experiments (either HTHP or arc-melting method) are all above 1020 K, so that only AlB2-type MnB2 can be synthesized. Therefore, it is essential to control the temperature accurately to synthesize the ReB2-type MnB2. On the other hand, the pressure should be controlled to be as low as possible. Further analyses show that the thermodynamic stability of MnB2 at high temperature mostly depends on the vibration frequency of Mn atoms. The stronger interactions between Mn and B in the ReB2-type MnB2 induce the vibration frequencies of Mn atoms shift to higher and increase the Gibbs free energy, causing the thermodynamics instability of ReB2-type MnB2 at high temperature. Therefore, there is no ReB2-type MnB2 synthesized at the temperature higher than 1020 K.

A new high-pressure polar phase of crystalline bromoform: a first-principles study.

The high-pressure phases of bromoform at zero temperature have been investigated by first-principles pseudopotential plane-wave calculations based on the density functional theory. A new high-pressure polar phase, ε, with space group CC has been found after a series of simulated annealing and geometry optimizations. Our calculated enthalpies showed that the transition from β phase to γ phase occurs at 1 GPa, then the γ phase transforms to the ε phase at 90 GPa. In addition, the Br···Br and C-H···Br interactions are the key factors for the polar aggregation in the ε phase. Further calculations show that the insulate-metal transition in ε phase due to band overlap happens at ~130 GPa.