Pressure-induced high-temperature superconductivity retained without pressure in FeSe single crystals.

To raise the superconducting-transition temperature (Tc) has been the driving force for the long-sustained effort in superconductivity research. Recent progress in hydrides with Tcs up to 287 K under pressure of 267 GPa has heralded a new era of room temperature superconductivity (RTS) with immense technological promise. Indeed, RTS will lift the temperature barrier for the ubiquitous application of superconductivity. Unfortunately, formidable pressure is required to attain such high Tcs. The most effective relief to this impasse is to remove the pressure needed while retaining the pressure-induced Tc without pressure. Here, we show such a possibility in the pure and doped high-temperature superconductor (HTS) FeSe by retaining, at ambient pressure via pressure quenching (PQ), its Tc up to 37 K (quadrupling that of a pristine FeSe at ambient) and other pressure-induced phases. We have also observed that some phases remain stable without pressure at up to 300 K and for at least 7 d. The observations are in qualitative agreement with our ab initio simulations using the solid-state nudged elastic band (SSNEB) method. We strongly believe that the PQ technique developed here can be adapted to the RTS hydrides and other materials of value with minimal effort.

Existence of BeCN2 and Its First-Principles Phase Diagram: Be and C Introducing Structural Diversity.

The existence and structure of BeCN2, the lightest representative of II-IV-V2 compounds, have for long remained unsolved, although previous theoretical studies have relied on assuming chemical similarity toward the known wurtzite-type BeSiN2. To solve the BeCN2 puzzle, we have now explored its potential-energy surface and here predict two additional polymorphs with space groups Cmc21 (porous phase) and Pmc21 (graphitic phase) in addition to another I4̅m2 type (carbodiimide-like), which is only slightly higher in energy than the wurtzite type. The phase diagram constructed from density-functional theory shows the Cmc21-type to be the ground state, stable in terms of the Gibbs energy under standard conditions, whereas the Pmc21- and I4̅m2-types are high-temperature phases; the wurtzite type, however, is the high-pressure phase. The kinetic barrier between the porous and graphitic phases is small, about 4 kJ mol-1, but larger toward the carbodiimide type, 25 kJ mol-1, and the wurtzite type, 28 kJ mol-1. Chemical-bonding analysis further reveals how beryllium and carbon induce structural diversity. As regards the second-lowest Pmc21-type, a monolayer of such graphitic BeCN2 shows the potential of photoelectrochemical water splitting, while a bilayer configuration should exhibit ferroelectricity with a polarization of 0.75 pC m-1. Further electronic-structure data of the four polymorphs indicate their potential for nonlinear optics.

Quasi-One-Dimensional Metallicity in Compressed CsSnI3.

Low-dimensional metal halides exhibit strong structural and electronic anisotropies, making them candidates for accessing unusual electronic properties. Here, we demonstrate pressure-induced quasi-one-dimensional (quasi-1D) metallicity in δ-CsSnI3. With the application of pressure up to 40 GPa, the initially insulating δ-CsSnI3 transforms to a metallic state. Synchrotron X-ray diffraction and Raman spectroscopy indicate that the starting 1D chain structure of edge-sharing Sn-I octahedra in δ-CsSnI3 is maintained in the high-pressure metallic phase while the SnI6 octahedral chains are distorted. Our experiments combined with first-principles density functional theory calculations reveal that pressure induces Sn-Sn hybridization and enhances Sn-I coupling within the chain, leading to band gap closure and formation of conductive SnI6 distorted octahedral chains. In contrast, the interchain I...I interactions remain minimal, resulting in a highly anisotropic electronic structure and quasi-1D metallicity. Our study offers a high-pressure approach for achieving diverse electronic platforms in the broad family of low-dimensional metal halides.

Large bandgap of pressurized trilayer graphene.

Graphene-based nanodevices have been developed rapidly and are now considered a strong contender for postsilicon electronics. However, one challenge facing graphene-based transistors is opening a sizable bandgap in graphene. The largest bandgap achieved so far is several hundred meV in bilayer graphene, but this value is still far below the threshold for practical applications. Through in situ electrical measurements, we observed a semiconducting character in compressed trilayer graphene by tuning the interlayer interaction with pressure. The optical absorption measurements demonstrate that an intrinsic bandgap of 2.5 ± 0.3 eV could be achieved in such a semiconducting state, and once opened could be preserved to a few GPa. The realization of wide bandgap in compressed trilayer graphene offers opportunities in carbon-based electronic devices.

Synthesis of Atomically Thin Hexagonal Diamond with Compression.

Atomically thin diamond, also called diamane, is a two-dimensional carbon allotrope and has attracted considerable scientific interest because of its potential physical properties. However, the successful synthesis of a pristine diamane has up until now not been achieved. We demonstrate the realization of a pristine diamane through diamondization of mechanically exfoliated few-layer graphene via compression. Resistance, optical absorption, and X-ray diffraction measurements reveal that hexagonal diamane (h-diamane) with a bandgap of 2.8 ± 0.3 eV forms by compressing trilayer and thicker graphene to above 20 GPa at room temperature and can be preserved upon decompression to ∼1.0 GPa. Theoretical calculations indicate that a (-2110)-oriented h-diamane is energetically stable and has a lower enthalpy than its few-layer graphene precursor above the transition pressure. Compared to gapless graphene, semiconducting h-diamane offers exciting possibilities for carbon-based electronic devices.

An effective structure prediction method for layered materials based on 2D particle swarm optimization algorithm.

A structure prediction method for layered materials based on two-dimensional (2D) particle swarm optimization algorithm is developed. The relaxation of atoms in the perpendicular direction within a given range is allowed. Additional techniques including structural similarity determination, symmetry constraint enforcement, and discretization of structure constructions based on space gridding are implemented and demonstrated to significantly improve the global structural search efficiency. Our method is successful in predicting the structures of known 2D materials, including single layer and multi-layer graphene, 2D boron nitride (BN) compounds, and some quasi-2D group 6 metals(VIB) chalcogenides. Furthermore, by use of this method, we predict a new family of mono-layered boron nitride structures with different chemical compositions. The first-principles electronic structure calculations reveal that the band gap of these N-rich BN systems can be tuned from 5.40 eV to 2.20 eV by adjusting the composition.

Cesium-mediated electron redistribution and electron-electron interaction in high-pressure metallic CsPbI3.

Electron-phonon coupling was believed to govern the carrier transport in halide perovskites and related phases. Here we demonstrate that electron-electron interaction enhanced by Cs-involved electron redistribution plays a direct and prominent role in the low-temperature electrical transport of compressed CsPbI3 and renders Fermi liquid (FL)-like behavior. By compressing δ-CsPbI3 to 80 GPa, an insulator-semimetal-metal transition occurs, concomitant with the completion of a slow structural transition from the one-dimensional Pnma (δ) phase to a three-dimensional Pmn21 (ε) phase. Deviation from FL behavior is observed upon CsPbI3 entering the metallic ε phase, which progressively evolves into a FL-like state at 186 GPa. First-principles density functional theory calculations reveal that the enhanced electron-electron coupling results from the sudden increase of the 5d state occupation in Cs and I atoms. Our study presents a promising strategy of cationic manipulation for tuning the electronic structure and carrier scattering of halide perovskites at high pressure.

Exotic Hydrogen Bonding in Compressed Ammonia Hydrides.

Hydrogen-rich compounds attract significant fundamental and practical interest for their ability to accommodate diverse hydrogen bonding patterns and their promise as superior energy storage materials. Here, we report on an intriguing discovery of exotic hydrogen bonding in compressed ammonia hydrides and identify two novel ionic phases in an unusual stoichiometry NH7. The first is a hexagonal R3̅ m phase containing NH3-H+-NH3, H-, and H2 structural units stabilized above 25 GPa. The exotic NH3-H+-NH3 unit comprises two NH3 molecules bound to a proton donated from a H2 molecule. Above 60 GPa, the structure transforms to a tetragonal P41212 phase comprising NH4+, H-, and H2 units. At elevated temperatures, fascinating superionic phases of NH7 with part-solid and part-liquid structural forms are identified. The present findings advance fundamental knowledge about ammonia hydrides at high pressure with broad implications for studying planetary interiors and superior hydrogen storage materials.

Materials discovery via CALYPSO methodology.

The structure prediction at the atomic level is emerging as a state-of-the-art approach to accelerate the functionality-driven discovery of materials. By combining the global swarm optimization algorithm with first-principles thermodynamic calculations, it exploits the power of current supercomputer architectures to robustly predict the ground state and metastable structures of materials with only the given knowledge of chemical composition. In this Review, we provide an overview of the basic theory and main features of our as-developed CALYPSO structure prediction method, as well as its versatile applications to design of a broad range of materials including those of three-dimensional bulks, two-dimensional reconstructed surfaces and layers, and isolated clusters/nanoparticles or molecules with a variety of functional properties. The current challenges faced by structure prediction for materials discovery and future developments of CALYPSO to overcome them are also discussed.