Search

The energy landscape of helium-nitrogen mixtures is explored by ab initio evolutionary searches, which predicted several stable helium-nitrogen compounds in the pressure range from 25 to 100 GPa. Among these high-pressure compounds, the monoclinic structure of ${\mathrm{HeN}}_{22}$ consists of neutral He atoms, partially ionic dimers ${\mathrm{N}}_{2}^{\ensuremath{\delta}\ensuremath{-}}$, and lanternlike cages ${\mathrm{N}}_{20}^{\ensuremath{\delta}+}$, showing a certain amount of charge transfer within the nitrogen framework. The monoclinic (${\mathrm{HeN}}_{20}{)}^{\ensuremath{\delta}+}{\mathrm{N}}_{2}^{\ensuremath{\delta}\ensuremath{-}}$ may be quenchable to ambient pressure with the estimated energy density of 10.44 kJ/g, which is $\ensuremath{\sim}2.4$ times larger than that of trinitrotoluene, indicating a very promising high-energy-density material.

Ordered borides of group IB and IIB metals (Cu, Ag, Au, Zn, Cd, Hg), as the well-known immiscible materials, are practically unknown due to the small electronegativity difference and large mismatch in their atomic sizes. Nevertheless, such rule may be broken under extreme conditions, i.e., high pressure or low dimensionality. Here two-dimensional (2D) copper borides were predicted from ab initio evolutionary searches, identifying that two structures are metallic whereas another one is strikingly a nodal line semimetal. These results challenge the long lasting puzzle in the immiscible systems and add new members to the 2D materials.

The work was supported by the National Natural Science Foundation of China (Grants No. 11604290 and No. 51732010), National Key R & D Program of China (Grant No. 2018YFA0703400), Funding Program for Recruited Oversea Scholars of Hebei Province (Grant No. CL201729), the Ph.D. Foundation by Yanshan University (Grant No. B970), and the Natural Science Research Project of Education Department of Anhui Province (KJ2018A0342). A.B. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness (FIS2016-76617-P) and the Department of Education, Universities and Research of the Basque Government and the University of the Basque Country (IT756-13).

Creating carrier reservoirs in layered compounds can effectively tune the carrier density, which often induces a variety of emergent properties. Based on solid-ion-conductor gating technique, we successfully induce superconductivity of 4.8 K in ultrathin Li-intercalated ${\mathrm{SnSe}}_{2}$ samples. The ${\mathrm{Li}}^{+}$ ions are driven in between interspacing of ${\mathrm{SnSe}}_{2}$ layers and form a single reservoir layer to provide electrons. In addition, a domelike ${T}_{c}$ is found through substituting S for Se, where the optimal ${T}_{c}$ is 6.2 K for ${\mathrm{SnSe}}_{1.8}{\mathrm{S}}_{0.2}$. Density-functional theory calculations confirm that the intercalated ${\mathrm{LiSnSe}}_{2}$ is thermodynamically favorable, where the intercalation of Li expands the interlayer spacing by 10% and increases the carrier density by two orders of magnitude. Meanwhile, the calculated results reveal that the enhanced electron-phonon interaction due to softened phonons determines the occurrence of superconductivity. Our results demonstrate that this strategy is very effective to explore superconductors in layered materials with narrow band gaps.

The icosahedron-based bulk boron structures have extremely chemical and structural complexity, and are usually semiconductors at ambient conditions. Here we predict bulk boron phases with a 60-atom orthorhombic unit cell from an ab initio evolutionary structure search, termed as ${\mathrm{B}}_{60}$. The metastable structures can be either a conductor or a semimetal depending on their interstitial atomic positions. In particular, an orthorhombic structure with $Pnma$ symmetry ($Pnma\ensuremath{-}{\mathrm{B}}_{60}$), consisting of ${\mathrm{B}}_{12}$ icosahedra and twisted interstitial two-atom wide boron ribbons, is identified to be a topological node-line semimetal with potential superior electronic transport. The band structure and simulated electron-diffraction pattern of $Pnma\ensuremath{-}{\mathrm{B}}_{60}$ are in satisfactory agreement with the experimental data, suggesting that it may exist in the form of nanomaterials.

Zinc-blende (zb) boron arsenide (BAs) has been confirmed to have impressively high thermal conductivity. However, studies on its phase transitions under pressure have been few. Here, through a recently developed structure search method, we predicted that many polytypes with the structural features of cubic and hexagonal diamond, which are known to be unstable even up to very high pressures for carbon and boron nitride, can become stable at low pressures and might be retained to ambient conditions. Moreover, some of these BAs polytypes are calculated to have impressively high thermal conductivities at ambient conditions and the thermal conductivities for zb- and $2H$-BAs will decrease with increasing pressure, which are mainly attributed to the stronger third anharmonic interaction. The current study will open up a route to the search for high thermal conductors.

A computation methodology based on ab initio evolutionary algorithms and the spin-polarized density functional theory was developed to predict two-dimensional (2D) magnetic materials. Its application to a model system borophene reveals an unexpected rich magnetism and polymorphism. A stable borophene with nonzero thickness was an antiferromagnetic (AFM) semiconductor from first-principles calculations, which can be further turned into a half metal by finite electron doping. In this borophene, the buckling and coupling among three atomic layers are not only responsible for the magnetism, but also result in an out-of-plane negative Poissons ratios under uniaxial tension, making it the first elemental material possessing auxetic and magnetic properties simultaneously., Comment: 8 pages, 8 figures

The high-pressure phases and superconductivity of ${\mathrm{CaYH}}_{12}$ have been explored by using a particle swarm optimization structure prediction methodology in combination with first-principles calculations. Our results show that ${\mathrm{CaYH}}_{12}$ becomes stable with a cubic $Fd\overline{3}m$ structure above 170 GPa, where metal atoms form body-centered-cubic (bcc) lattices and hydrogens occupy all the tetrahedral interstices of these bcc lattices, completing sodalitelike cages. The electron-phonon coupling calculations indicate that the $Fd\overline{3}m$ structure is a potential high-temperature superconductor, with a calculated ${T}_{c}$ of 258 K at 200 GPa. Our current study provides a possibility for searching new high-${T}_{c}$ superconductors in ternary hydrides.

Using our newly developed particle swarm optimization algorithm on crystal structural prediction, we characterized the pressure-induced structural transition sequence of gallane (GaH3). As has been observed in alane (AlH3), enthalpy calculations reveal that the Pm3̄n structure of GaH3 becomes stable above 160 GPa, below which it is unstable with respect to elemental decomposition. Interestingly, the Pm3̄n structure is metallic, and the application of the Allen-Dynes modified McMillan equation reveals a high superconducting transition temperature (Tc), which reaches 86 K at 160 GPa and increases with decreasing pressure (Tc = 102 K at 120 GPa). Our band structure calculations demonstrate that GaH 3 within the Pm3̄n structure is a highly ionic solid, where the ionicity of H atoms plays an important role in the predicted high temperature superconductivity. © 2011 American Physical Society.

Recent high-pressure synthesis of ${\text{H}}_{2}$-rich van der Waals compounds have attracted great interests in probing novel ${\text{H}}_{2}$ physics, which in general remain elusive. We have solved the crystal structure of the synthesized ${\text{SiH}}_{4}{({\text{H}}_{2})}_{2}$ by first-principles calculations and revealed that ${\text{SiH}}_{4}$ molecules in the formation of ${\text{SiH}}_{4}{({\text{H}}_{2})}_{2}$ remain nearly unaltered with Si atoms forming a peculiar tetragonal lattice, which can also be viewed as a distorted face-centered-cubic lattice. We have provided direct evidences on that ${\text{H}}_{2}$ molecules in ${\text{SiH}}_{4}{({\text{H}}_{2})}_{2}$ occupy the interstitial sites, and more intriguingly are orientationally disordered. Our argument has been supported by the excellent mutual agreement between theoretical and experimental equation of states, Raman, and x-ray diffraction data. The current study has strong implications on other high-pressure van der Waals compounds, e.g., ${\text{H}}_{2}{\text{O-H}}_{2}$, ${\text{CH}}_{4}{\text{-H}}_{2}$, ${\text{NH}}_{3}{\text{BH}}_{3}{\text{-H}}_{2}$, ${\text{Ar-H}}_{2}$, and ${\text{Xe-H}}_{2}$.

Using the first-principles full-potential linearized augmented plane-wave method based on density functional theory, we have investigated the electronic structure and magnetism of hypothetical MC (M=Mg, Ca, Sr, and Ba) compounds with the zinc-blende (ZB) crystal structure. It is shown that ZB CaC, SrC, and BaC are half-metallic ferromagnets with large half-metallic gaps (up to 0.83 eV). The half metallicity is found to be robust with respect to the lattice compression and is maintained up to the lattice-constant contraction of 14%, 13%, and 9% for CaC, SrC, and BaC, respectively. The exchange interactions in these compounds are studied using the augmented spherical wave method in conjunction with the frozen-magnon approach. The Curie temperature is estimated within both the mean field approximation and the random phase approximation. The predicted Curie temperatures of all three half-metallic compounds considerably exceed the room temperature. The large half-metallic gaps, the robustness of the half metallicity with respect to the lattice contraction, and the high Curie temperatures make these systems interesting candidates for applications in spintronic devices. The absence of the transition-metal atoms makes these compounds important model systems for the study of the origin and properties of the half-metallic ferromagnetism of s-p electron systems.

We performed systematic full-potential density functional theory studies on all possible (1x1) terminations of the low-index surface (111) of FeO with NaCl (B1) phase and inverse NiAs (iB8) phase, respectively. Applying the concept of first-principles atomistic thermodynamics, we analyze the composition, structure, and stability of the FeO (111) orientation in equilibrium with an arbitrary oxygen environment. The density of states (DOS) of the relaxed FeO (111) surface with B1 structure and inverse B8 (iB8) structure within the studied subset of (1x1) geometries were calculated and compared with the DOS of the bulk FeO with the two structures. The calculations reveal that the (111) surfaces of FeO(B1)-Fe and FeO(B1)-O have metallic and ferromagnetic properties, and they are different from those of the bulk of FeO (B1). While the (111) surfaces of FeO(iB8)-Fe and FeO(iB8)-O show semiconducting and antiferromagnetic properties similar to those of the bulk of FeO(iB8).