Exploration of stable compounds, crystal structures, and superconductivity in the Be-H system

Using first-principles variable-composition evolutionary methodology, we explored the high-pressure structures of beryllium hydrides between 0 and 400 GPa. We found that BeH$_2$ remains the only stable compound in this pressure range. The pressure-induced transformations are predicted as $Ibam$ $\rightarrow$ $P\bar{3}m1$ $\rightarrow$ $R\bar{3}m$ $\rightarrow$ $Cmcm$ $\rightarrow$ $P4/nmm$, which occur at 24, 139, 204 and 349 GPa, respectively. $P\bar{3}m1$ and $R\bar{3}m$ structures are layered polytypes based on close packings of H atoms with Be atoms filling octahedral voids in alternating layers. $Cmcm$ and $P4/nmm$ structures have 3D-networks of strong bonds, but also feature rectanular and squre, respectively, layers of H atoms with short H-H distances. $P\bar{3}m1$ and $R\bar{3}m$ are semiconductors while $Cmcm$ and $P4/nmm$ are metallic. We have explored superconductivity of both metallic phases, and found large electron-phonon coupling parameters of $\lambda$=0.63 for $Cmcm$ (resulting in a $T_c$ of 32.1-44.1 K) at 250 GPa and $\lambda$=0.65 for $P4/nmm$ ($T_c$ = 46.1-62.4 K) at 400 GPa. The dependence of $T_c$ on pressure indicates that $T_c$ initially increases to a maximum of 45.1 K for $Cmcm$ at 275 GPa and 97.0 K for $P4/nmm$ at 365 GPa, and then decreases with increasing pressure for both phases

Phase stability, chemical bonding and mechanical properties of titanium nitrides: A first-principles study

We have performed first-principles evolutionary searches for all stable titanium nitrides and have found, in addition to the well-known rocksalt-type TiN, new ground states Ti$_3$N$_2$, Ti$_4$N$_3$, Ti$_6$N$_5$ at atmospheric pressure, and Ti$_2$N and TiN$_2$ at higher pressures. The latest nitrogen-rich structure presents encapsulated N$_2$ dumbbells with a N-N distance of 1.348 {\AA} at 60 GPa and TiN$_2$ is predicted to be mechanically stable (quenchable). Our calculations of the mechanical properties (bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and hardness) are in excellent agreement with the available experimental data and show that the hardness of titanium nitrides increases with increasing nitrogen content. The hardness of titanium nitrides is enhanced by strengthening directional covalent bonds and disappearance of Ti-Ti metallic bonds. Among the predicted compounds, TiN$_2$ has the highest hardness of 27.2 GPa.Comment: 8 pages, 7 figure

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 parent pool of an evolutionary crystal structure search. The effectiveness of this approach is benchmarked on three known metastable systems: XeN$_8$, with a two-dimensional polymeric nitrogen sublattice, brookite TiO$_2$, and a high pressure BaH$_4$ phase that was recently characterized. Additionally, a newly predicted metastable melaminate salt, $P$-1 WC$_{3}$N$_{6}$, was found to possess an energy that is lower than 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 developing new synthesis targets with desired properties

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

Barium–Nitrogen Phases Under Pressure: Emergence of Structural Diversity and Nitrogen-Rich Compounds

International audienceAlthough the potential of polynitrogen as a high-energy density material (HEDM) has attracted attention, the difficulty of preserving polynitrogen thwarts attempts to discover molecular and extended nitrogen structures. Mixing nitrogen with electropositive elements to obtain viable solid-state compounds represents one approach to overcome thermodynamic/kinetic instability. In pursuit of barium nitrides within the Ba–N family, we theoretically explored the ground/metastable structures from ambient pressure up to 100 GPa. Crystal structure prediction (CSP) based on evolutionary algorithms and density functional theory identified 13 stoichiometries and 24 stable structures; several metastable phases were dynamically stable. Pressure and barium/nitrogen ratio represent controllable factors for polynitrogen net preparation. Four types of phases could be classified based on nitrogen structural dimensionality: isolated nitrogen atom; nitrogen molecules, e.g., N2 dumbbells, linear N3 azides, N4 zigzag units, N5 pentazolate, N6 six-membered rings; 1D polythiazyl S2N2-like nitrogen chains; and 2D polymeric nitrogen layers. Interestingly, P63/mcm-Ba3N, R3̅m-Ba2N, and C2/m-Ba3N2 have predicted electride properties. Notably, we observe electronic property changes in the charge-balanced Ba3N2 compound as pressure increases. Solid-state Ba3N2 changes from a conducting electride at ambient pressure with encapsulated anionic N2 dumbbells and isolated N atoms to a nitride semiconductor above 5 GPa in which isolated N3– ions are trapped within a Ba2+ ocean—as expected for textbook charge-balanced structures—and is metallic above 25 GPa. In addition, ab initio molecular dynamics analysis indicates nitrogen-rich BaN2, BaN4, and bis-pentazolate Ba(N5)2 are quenchable to ambient pressure, suggesting these polymeric nitrogen networks can be preserved up to at least 600 K; these quenchable phases are promising candidate HEDMs

La Procédure D’Annulation des Sentences Arbitrales du Cirdi

Note:La Convention pour le règlement des différends relatifs aux investissements entre États et ressortissants d'autres États a institué un système de conciliation et d'arbitrage, à la fois spécialise, autonome et exclusif, lequel est administre par le centre international pour le règlement des différends relatifs aux investissements (“CIRDI"). La Convention de Washington de 1965 ne prévoit que trois voies de recours à l'encontre des sentences. Elles doivent être exercées dans le cadre de la Convention, à I' exclusion des recours devant les juridictions nationales. Le recours en annulation devait être employé dans des circonstances exceptionnelles. Toutefois, le succès obtenu par plusieurs requetés en annulation a suscité des doutes sur l'efficacité du système du CIRDI. Alors que les rédacteurs de la Convention voulaient distinguer le recours en annulation d'un appel, les instances de contrôle ont procédé à une révision extensive du bien-fondé des sentences.The Convention on the Settlement of Investment Disputes between States and Nationals of Other States established a specialized autonomous and self-contained conciliation and arbitration system administered by the International Centre for Settlement of Investment Disputes, an international institution created by the Convention ("ICSID"). The Washington Convention of 1965 provides only for three remedies, which can be invoked against awards. They must be exercised within the framework of the Convention, the intervention of national courts being excluded. The remedy of annulment was intended to be used in exceptional circumstances. However, the success of several annulment proceedings raised serious concern about the effectiveness of the ICSID system. Although the drafters of the Convention wanted to distinguish an annulment proceeding from an appeal, the review panels were drawn into a detailed review of the merits of the awards