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Motivated by searching room-temperature superconductors that could be realized near ambient conditions, palladium hydrides were chosen as the research subject considering that they can stably exist under ambient conditions, and Li as an electron donor for its outstanding performance in chemically tuning stability. A novel cubic phase structure of Li2PdH6 with a remarkably high estimated Tc of ∼165 K at 90 GPa was found using particle swarm optimization algorithm calculations. The superconducting behavior persists down to 10 GPa with a high Tc of 106.382 K. Even though the parent binary Pd–H system is not a good superconductor, the introduction of extra electrons breaks up the H2 molecules, inducing the increase of atomic hydrogen compared with parent hydride, which is necessary for outstanding superconducting behavior. The existence of relatively soft phonons associated with the H atoms in phonon dispersion curves is responsible for its high-Tc. Our results indicated that the doping of Li to binary hydrides, especially to binary hydrides with low-Tc that exist under ambient pressure, can produce robust phonon-mediated superconductivity. This may be a strategy to design and optimize room-temperature superconductors that can be synthesized under modest pressure. The findings may pave the way for realizing new high-Tc superconductors in experiments under lower pressure than recently documented superconducting hydrides. [ABSTRACT FROM AUTHOR]

Pressure-induced vibrational and superconducting properties of lanthanum hydrides (La[H.sub.2] and La[H.sub.3] have been studied using first principles calculations. It is found that La[H.sub.2]and La[H.sub.3] are dynamically stable in the pressure ranges of 0-39 GPa and 4-35 GPa, respectively. The character of phonon dispersion curves for La[H.sub.2]and La[H.sub.3] is analyzed under pressure. The zone-center phonon mode eigen displacements that represent infrared and Raman activity are also obtained, which are essential to the analysis of spectral experiments. The calculations based on Bardeen-Cooper-Schrieffer theory indicate that La[H.sub.2] almost has no superconducting behavior even under pressure, in reasonable agreement with previous theoretical calculations and experiments. Whereas, La[H.sub.3] presents a considerable high superconducting transition temperature ([T.sub.c]) at the onset of the face centered cubic structure, while it decreases exponentially under further compression up to 25 GPa and finally almost approaches zero. Further analysis indicates that the underlying mechanism of these two distinct superconducting behaviors are closely related to the hybridization between the [H.sub.O]-s state and La-d state. The mode Gruneisen parameters of two hydrides are also analyzed under 35 GPa, finding that the hydrogen atoms at octahedral sites are responsible for the superconducting properties of La[H.sub.3], and in fact, the unobserved superconducting behavior in La[H.sub.2]can be interpreted as the absence of hydrogen at octahedral sites compared with La[H.sub.3]. Key words: lattice dynamic properties, superconducting properties, mode Gruneisen parameters, vibration modes, superconducting mechanism. Nous faisons un calcul ab initio des proprietes vibrationnelles et supraconductrices induites par la pression dans les hydrides du lanthane La[H.sub.2] et La[H.sub.3]. Nous trouvons que La[H.sub.2] et La[H.sub.3] sont dynamiquement stables dans les domaines de pression 0-39 GPa et 4-35 GPa, respectivement. Nous analysons les courbes de dispersion des phonons pour les deux hydrides sous pression. Nous obtenons le mode de phonon en centre de zone sous deplacements propres qui contribuent aux activites infrarouge et Raman et jouent un role important dans l'analyse des experiences spectroscopiques. Des calcul en theorie Bardeen-Cooper-Schrieffer indiquent que La[H.sub.2] n'a pratiquement aucune propriete supraconductrice, meme sous pression, ce qui agree raisonnablement avec d'autres resultats theoriques et experimentaux. Par contre, La[H.sub.3] presente une haute temperature de transition supraconductrice ([T.sub.c]) lors qu'apparait la structure cubique a face centree, mais qui baisse de facon exponentielle si la pression continue d'augmenter jus qu'a 25 GPa, pour finalement approcher zero. Une analyse plus poussee indique que le mecanisme sous-jacent a ces deux comportements supraconducteurs differents est relie a l'hybridation entre les etats [H.sub.O]-s et La-d. Nous etudions aussi les parametres de Gruneisen des deux hydrides sous une pression de 35 GPa, trouvant que les atomes d'hydrogene aux sites octahedriques de La[H.sub.3] sont responsables ses proprietes supraconductrices et, de fait, l'absence de supraconductivite dans le La[H.sub.2] peut etre interprete comme due a l'absence d'hydrogene dans ses sites octahedriques. [Traduit par la Redaction] Mots-cles : proprietes de dynamique du reseau, proprietes supraconductrices; parametres de Gruneisen, modes de vibration, mecanisme supraconducteur. PACS Nos.: 63.50.-x, 63.22.Rc, 74.20.Fg, 74.81.Bd., 1. Introduction Metal hydrides are of great scientific and technological interest in view of their fantastic properties and potential applications, such as high-energetic, high-[T.sub.c] , and hydrogen storage [1-3]. Generally, [...]

The structure, phase transition, elastic, and thermodynamic properties of Os[N.sub.2] have been studied via first-principles calculations. It is shown that the CoS[b.sub.2] structure is more stable than other structures. By the calculated H-P relations at 0 K, we found that the phase transition of Os[N.sub.2] from CoS[b.sub.2] structure to marcasite structure (ζ → δ) occurs at 16.8 GPa, while the phase transition pressure between pyrite structure and fluorite structure (γ → α) is 80 GPa. The results of obtained phase transitions are also confirmed by bond length, sound velocity, and thermal expansion coefficient under different pressures. The pressure dependences of the elastic constants, mechanical stability, and mechanical anisotropy of four structures of Os[N.sub.2] have been investigated by finding that the fluorite (pyrite and marcasite) structure Os[N.sub.2] is mechanically stable under hydrostatic pressure (up to 60 GPa). However, the monoclinic structure is mechanically unstable under pressure from 0 to 60 Gpa. The calculated elastic anisotropic factors show that Os[N.sub.2] possesses high elastic anisotropy under pressure. Moreover, the calculations on total density of states show that Os[N.sub.2] of different structures has a metallic character, in agreement with previous theoretical results. The thermodynamic properties and sound velocity under diverse pressures of Os[N.sub.2] of the four structures have been also investigated successfully. PACS Nos.: 31.15.A-, 36.40.Ei, 87.19.rd. Nous presentons une etude ab initio de la structure, de la transition de phase et des proprietes thermodynamiques et elastiques de Os[N.sub.2]. Nous montrons que la structure de type CoS[b.sub.2] est pus stable que les autres structures. A l'aide des relations H-P calculees, nous trouvons que la transition de phase de Os[N.sub.2] de la structure CoS[b.sub.2] vers une structure marcassite (ζ → δ) se produit a 16.8 GPa, alors que la pression de transition de phase entre les structures pyrite et fluorine (γ → α) est 80 GPa. Les resultats obtenus sur les transitions de phase sont confirmes par la longueur de liaison, la vitesse du son et le parametre d'expansion thermique sous differentes pressions. Nous analysons d'abord la dependance sur la pression de la constante electrique, de la stabilite mecanique et de l'anisotropie mecanique pour quatre structures de Os[N.sub.2], trouvant que la structure fluorine (pyrite et marcassite) de Os[N.sub.2] est mecaniquement stable sous pression hydrostatique (jusqu'il 60 GPa). Cependant, la structure monoclinique est mecaniquement instable sous des pressions de 0 a 60 GPa. Les facteurs d'anisotropie elastique calcules pour Os[N.sub.2] possedent une haute anisotropie elastique sous pression. De plus, le calcul de la densite totale d'etats de differentes structures montre que Os[N.sub.2] a un caractere metallique, en accord avec des resultats theoriques anterieurs. Nous investiguons avec succes les proprietes thermodynamiques et la vitesse du son sous diverses pressions pour quatre structures de Os[N.sub.2]. [Traduit par le Redaction], 1. Introduction Recently, transition metal nitrides have attracted special attention for their unique properties such as hardness, superconductivity, and durability [1-5]. Several experimental investigations have been performed to explore the [...]

Hydrogen-rich systems are currently thought to constitute the most promising potential high-temperature superconductor materials. Here, the high-pressure structure and superconductivity of the ternary hydrogen-rich system ScCaH2n (n = 1–6) are systematically investigated by using the prediction method of particle swarm optimization structure combined with first-principles calculations. As n increases, the electron local function (ELF) indicates that the hydrogen atoms in this system exhibit different behaviors corresponding to single H atoms, H2 molecules, graphene-like layers and, ultimately, H clathrate cages. The electron phonon coupling (EPC) calculation shows that the superconducting transition temperature for the hydrogen cage structures is much higher than that of other structures, which is mainly attributed to the increasing contribution of H in the cage structure to the density of states (DOS) at the Fermi level. With the ScCaH2n system we have found two potential high temperature superconductor structures: ScCaH8 and ScCaH12. For these two compounds, the corresponding Tc values reach around 212 K and 182 K, respectively, at 200 GPa. The ScCaH8 structure exhibits an H18-cage in which the length of four hydrogen bonds increases abnormally with increasing pressure, leading to a decrease in the Tc value after 200 GPa. The present work therefore demonstrates an unusual pressure-induced behavior and provides new ideas to guide the search for new high temperature superconductors in ternary hydrides. [ABSTRACT FROM AUTHOR]

Ternary metal hydrides play an essential role in the search for conventional high‐temperature superconductors because they can be synthesized under mild conditions and recovered at ambient pressure. It has been widely accepted that the electronic structure, metallization pressure, and superconducting behavior of binary hydrides can be adjusted effectively by doping, replacing, or introducing a new element. In this work, yttrium hydrides were chosen as parent hydrides, while scandium was considered the doping element to perform systematical crystal structure searches on the Sc‐Y‐H system under pressure. A new ternary hydride ScYH6 with a Pm‐3 structure (cP8) was found below 150 GPa according to Particle Swarm Optimization calculations, and then, a P4/mmm phase (tP8) becomes favorable from 150 GPa. Importantly, cP8‐ScYH6 is dynamically stable under pressure as low as 0.01 GPa with a superconducting temperature (Tc) of 32.110 K for Coulomb pseudopotential μ* = 0.13, indicating that ternary hydrides are promising candidates in the search for superconductors that can be synthesized under mild conditions in hydrogen‐rich materials. The analysis using the "triangle straight‐line method", compared with enthalpy difference calculations, showed that the most reasonable synthesis pathway of ScYH6 is ScH3 + YH3 → ScYH6 in the whole pressure regime studied in this work. The Tc of ScYH6 has a linear relationship with pressure up to 52.907 K under 200 GPa. The lattice dynamical calculations demonstrate that the H atoms in both cP8 and tP8 structures make crucial contributions to the superconducting behavior of ScYH6. These findings can further reveal the influence of doping, replacing, and introducing element on the superconducting behavior of binary hydrides. [ABSTRACT FROM AUTHOR]

We have performed quantum-based multiscale simulationsto study the initial chemical processes of condensed-phase octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine(HMX) under shock wave loading. A self-consistent charge density-functionaltight-binding (SCC-DFTB) method was employed. The results show thatthe initial decomposition of shocked HMX is triggered by the N–NO2bond breaking under the low velocity impact (8 km/s). Asthe shock velocity increases (11 km/s), the homolytic cleavage ofthe N–NO2bond is suppressed under high pressure,the C–H bond dissociation becomes the primary pathway for HMXdecomposition in its early stages. It is accompanied by a five-memberedring formation and hydrogen transfer from the CH2groupto the −NO2group. Our simulations suggest thatthe initial chemical processes of shocked HMX are dependent on theimpact velocity, which gain new insights into the initial decompositionmechanism of HMX upon shock loading at the atomistic level, and haveimportant implications for understanding and development of energeticmaterials. [ABSTRACT FROM AUTHOR]

The lattice dynamic, elastic, superconducting, and thermodynamic properties of the high-pressure cubic metallic phase AlH3 are studied within density function theory. The calculated elastic modulus and phonon dispersion curves at various pressures indicate that the cubic phase is both mechanically and dynamically stable above 73 GPa. The superconducting transition temperature was calculated using Allen-Dynes modification of the McMillan formula based on the Bardeen-Cooper-Schrieffer theory. It is found that Tc approaches a linear decrease in the low pressure range at the rate dTC/dP≈-0.22 K/GPa but gradually decreases exponentially at higher pressure, and then it becomes 0 K upon further compression. The calculations indicate that Tc is about 2.042 K at 110 GPa, in agreement with experimental results. The soft phonon modes, especially the lowest acoustic mode, contribute almost 79% to the total electron-phonon coupling parameter sλ for cubic AlH3 at 73 GPa. However, they disappear gradually with increasing pressure, showing a responsibility for the variation of Tc. The thermodynamic properties of cubic AlH3, such as the dependence of thermal expansion coefficient αV on pressure and temperature, the specific heat capacity CP, as well as the electronic specific heat coefficient Cel, were also investigated by the quasi-harmonic approximation theory. [ABSTRACT FROM AUTHOR]