Predicted High-Energy Density MN8 Containing Anionic 18-Crown-6 Ring-Based Polynitrogen Monolayers Acting as Cryptand.

In this study, we investigate the potential of the 18-crown-6-like two-dimensional (2D)-N8 structure to accommodate electrons from metals without compromising its covalent nitrogen network. Employing the crystal structure prediction enhanced by evolutionary algorithm and density functional theory methodology, we successfully predicted the existence of 16 layered M@2D-N8 complexes from a total of 39 MN8 systems investigated at 100 GPa (M = s-block Na-Cs, Be-Ba and d-block Ag, Au, Cd, Hg, Hf, W, and Y). Among those, there are 13 quenchable M@2D-N8 compounds that are dynamically stable at 1 atm. Orbital interactions and bonding analysis show that 2D-N8 presents a flat localized π* band that can accommodate one or two electrons without breaking the 2D covalent nitrogen network. Depending on the metal-to-polynitrogen charge transfer (formally, 1-4 electrons), these N-rich phases are semiconducting or metallic under ambient conditions. Ab initio molecular dynamics simulations show that K(I)@2D-N8 and Ca(II)@2D-N8 are thermally stable up to 600 K, while the Hf(IV)@2D-N8 compound is thermally not viable at 400 K because of the weakening of the N═N bonds due to a strong four-electron reduction. These metal 18-crown-6 ring-based polynitrogen compounds, as expected due to their high nitrogen content (eight nitrogen atoms per metal), could potentially serve as new high-energy density materials.

Structurally Constrained Evolutionary Algorithm for the Discovery and Design of Metastable Phases.

Metastable materials are abundant in nature and technology, showcasing remarkable properties that inspire innovative materials design. However, traditional crystal structure prediction methods, which rely solely on energetic factors to determine a structure's fitness, are not suitable for predicting the vast number of potentially synthesizable phases that represent a local minimum corresponding to a state in thermodynamic equilibrium. Here, we present a new approach for the prediction of metastable phases with specific structural features and interface this method with the XtalOpt evolutionary algorithm. Our method relies on structural features that include the local crystalline order (e.g, the coordination number or chemical environment), and symmetry (e.g, Bravais lattice and space group) to filter the breeding pool of an evolutionary crystal structure search. The effectiveness of this approach is benchmarked on three known metastable systems: XeN8, with a two-dimensional polymeric nitrogen sublattice, brookite TiO2, and a high pressure BaH4 phase, which was recently characterized. Additionally, a newly predicted metastable melaminate salt, P1̅ WC3N6, was found to possess an energy that is lower than that of two phases proposed in a recent computational study. The method presented here could help in identifying the structures of compounds that have already been synthesized, and in developing new synthesis targets with desired properties.

Oxygen functionalized InSe and TlTe two-dimensional materials: transition from tunable bandgap semiconductors to quantum spin Hall insulators.

From first-principles calculations, we found that oxygen functionalized InSe and TlTe two-dimensional materials undergo the following changes with the increased concentrations of oxygen coverage, transforming from indirect bandgap semiconductors to direct bandgap semiconductors with tunable bandgap, and finally becoming quantum spin hall insulators. The maximal nontrivial bandgap are 0.121 and 0.169 eV, respectively, which occur at 100% oxygen coverage and are suitable for applications at room temperature. In addition, the topological phases are derived from SOC induced p-p bandgap opening, which can be further determined by Z2 topological invariants and topologically protected gapless edge states. Significantly, the topological phases can be maintained in excess of 75% oxygen coverage and are robust against external strain, making the quantum spin hall effect easy to achieve experimentally. Thus, the oxygen functionalized InSe and TlTe are fine candidate materials for the design and fabrication of topological devices.

New developments in the GDIS simulation package: Integration of VASP and USPEX.

A popular first principles simulation code, the Vienna Ab initio Simulation Package (VASP), and a crystal structure prediction (CSP) package, the Universal Structure Predictor: Evolutionary Xtallography (USPEX) have been integrated into the GDIS visualization software. The aim of this integration is to provide users with a unique and simple interface through which most of the steps of a typical crystal optimization or prediction work. This involved, for the latter, not only setting up a CSP calculation with complete support for the latest version of USPEX, but also displaying the many structure results by linking each structure geometry and its energy via interactive graphics. For the optimization part, any structure displayed by GDIS can now be the starting point for VASP calculations, with support for its most commonly used parameters. Atomic and electronic structures can be displayed as well as dynamic properties such as total energy, force, volume, and pressure for each ionic step. It is not only possible to start calculations from the GDIS visualization software, using an in-place task manager, but a running calculation can also be followed, allowing a greater control of the simulation process. The GDIS software is available under the GNU public license in its second version.

Prediction of a New Layered Polymorph of FeS2 with Fe3+S2-(S22-)1/2 Structure.

The never-elucidated crystal structure of metastable iron disulfide FeS2 resulting from the full deintercalation of Li in Li2FeS2 has been cracked thanks to crystal structure prediction searches based on an evolutionary algorithm combined with first-principles calculations accounting for experimental observations. Besides the newly layered C2/m polymorph of iron disulfide, two-dimensional dynamically stable FeS2 phases are proposed that contain sulfides and/or persulfide S2 motifs.

Copper halide diselenium: predicted two-dimensional materials with ultrahigh anisotropic carrier mobilities.

On the basis of first-principles calculations, we discuss a new class of two-dimensional materials-CuXSe2 (X = Cl, Br) nanocomposite monolayers and bilayers-whose bulk parent was experimentally reported in 1969. We show the monolayers are dynamically, mechanically and thermodynamically stable and have very small cleavage energies of ∼0.26 J m-2, suggesting their exfoliation is experimentally feasible. The monolayers are indirect-gap semiconductors with practically the same moderate band gaps of 1.74 eV and possess extremely anisotropic and very high carrier mobilities (e.g., their electron mobilities are 21 263.45 and 10 274.83 cm2 V-1 s-1 along the Y direction for CuClSe2 and CuBrSe2, respectively, while hole mobilities reach 2054.21 and 892.61 cm2 V-1 s-1 along the X direction). CuXSe2 bilayers are also indirect band gap semiconductors with slightly smaller band gaps of 1.54 and 1.59 eV, suggesting weak interlayer quantum confinement effects. Moreover, the monolayers exhibit high absorption coefficients (>105 cm-1) over a wide range of the visible light spectra. Their moderate band gaps, very high unidirectional electron and hole mobilities, and pronounced absorption coefficients indicate the proposed CuXSe2 (X = Cl, Br) nanocomposite monolayers hold significant promise for application in optoelectronic devices.