A first-principles study of theoretical superconductivity on RbH by doping without applied pressure.

The structural, electronic, lattice dynamics, electron-phonon (el-ph) coupling, and superconducting (SC) properties of the alkali-metal hydride RbH, metallized through electron-doping by the construction of the solid-solution Rb1-xSrxH, are systematically analyzed as a function of Sr-content within the framework of density functional perturbation and Migdal-Eliashberg theories, taking into account the effect of zero-point energy contribution by the quasi-harmonic approximation. For the entire studied range of Sr-content, steady increments of the el-ph coupling constant and the SC critical temperature are found with progressive alkaline-earth metal content through electron-doping, reaching the values ofλ = 1.92 andTc=51.3(66.1)K withµ∗= 0.1(0). The steady rise of such quantities as a function of Sr-content is consequence of the metallization of the hydride as an increase of density of states at the Fermi level is observed, as well as the softening of the phonon spectrum, mainly coming from H-optical modes. Our results indicate that electron-doping on metal-hydrides is an encouraging alternative to look for superconductivity without applied pressure.

Electron- and hole-doping on ScH2and YH2: effects on superconductivity without applied pressure.

We present the evolution of the structural, electronic, and lattice dynamical properties, as well as the electron-phonon (el-ph) coupling and superconducting critical temperature (Tc) of ScH2and YH2metal hydrides solid solutions, as a function of the electron- and hole-doping content. The study was performed within the density functional perturbation theory, taking into account the effect of zero-point energy through the quasi-harmonic approximation, and the solid solutions Sc1-xMxH2(M = Ca, Ti) and Y1-xMxH2(M = Sr, Zr) were modeled by the virtual crystal approximation. We have found that, under hole-doping (M = Ca, Sr), the ScH2and YH2hydrides do not improve their el-ph coupling properties, sensed byλ(x). Instead, by electron-doping (M = Ti, Zr), the systems reach a critical contentx≈ 0.5 where the latent coupling is triggered, increasingλas high as 70%, in comparison with itsλ(x= 0) value. Our results show thatTcquickly decreases as a function ofxon the hole-doping region, fromx= 0.2 tox= 0.9, collapsing at the end. Alternatively, for electron-doping,Tcfirst decreases steadily untilx= 0.5, reaching its minimum, but forx> 0.5 it increases rapidly, reaching its maximum value of the entire range at the Sc0.05Ti0.95H2and Y0.2Zr0.8H2solid solutions, demonstrating that electron-doping can improve the superconducting properties of pristine metal hydrides, in the absence of applied pressure.

Comparing two high correlation models to test the mechanical stability of americium-II.

In this work two high density functional theory (DFT) correlation methodologies, the so called DFT+U (or GGA+U) implementation and the exact exchange of correlated electrons (EECE), hybrid DFT functional (or one case of hybrid DFT), are tested to determine the mechanical properties of americium-II. For each case, the numeric value of their principal parameter is chosen ([Formula: see text] for the first case and [Formula: see text] for the second one) once the crystalline structure meets all the mechanical stability conditions. The results show that there is a range of values of [Formula: see text] and [Formula: see text] in which both methodologies generate a stable (experimentally correct) non-magnetic ground state, reaching approximately the same numeric value of the set of elastic constants of the cubic structure. However, only for the case of the hybrid functional results it is possible to show how the non-magnetic configuration is energetically favored, as compared to the ferromagnetic configuration. This happens around [Formula: see text], a value in agreement with a previous analysis made under the same methodology for the metal case Am-I. Following a detailed and deep analysis, it is possible to find a close interrelation between the electronic properties of the metal: its distribution of states around the Fermi level, the energy difference between the two possible spin configurations, and the mechanical response of the crystal. Also, it is possible to conclude that the effect of alpha parameter on the [Formula: see text] electrons can be used as a parameter to simulate the presence of an external pressure over the structure. For the comparison, the calculations were performed within the LAPW approximation in DFT as implemented in the WIEN2k code, with a finite deformation method.

Effect of doping on lattice dynamics and electron-phonon coupling of the actinides Ac-Th alloy.

We have studied the electronic, lattice dynamical, and electron-phonon properties of the actinides [Formula: see text]Th x alloy within the framework of density functional perturbation theory. The self-consistent virtual crystal approximation is used for the alloy modeling, and spin-orbit coupling is included in the calculation of all relevant quantities. An overall decrease of the electron-phonon coupling (λ) by [Formula: see text] from Ac to Th was observed. However, its dependence on x shows a non-linear behavior. λ reduces just 6% from Ac to a Th content of [Formula: see text], then drops drastically (∼[Formula: see text]) from there until [Formula: see text]. The large decrease of λ for [Formula: see text] is due to the reduction of the density of states at the Fermi level ([Formula: see text]), combined with a general phonon hardening. On contrast, the behavior for [Formula: see text] is the result of a subtle balance between an enhancement of phase space and the above mentioned effects on [Formula: see text] and the phonons. The phase-space enhancement is related to the appearance of Kohn anomalies, which fade away as the Th concentration increases.

Effects of electron doping on the stability of the metal hydride NaH.

Alkali and alkali-earth metal hydrides have high volumetric and gravimetric hydrogen densities, but due to their high thermodynamic stability, they possess high dehydrogenation temperatures which may be reduced by transforming these compounds into less stable states/configurations. We present a systematic computational study of the electron doping effects on the stability of the alkali metal hydride NaH substituted with Mg, using the self-consistent version of the virtual crystal approximation to model the alloy Na1-x Mg x H. The phonon dispersions were studied paying special attention to the crystal stability and the correlations with the electronic structure taking into account the zero point energy contribution. We found that substitution of Na by Mg in the hydride invokes a reduction of the frequencies, leading to dynamical instabilities for Mg content of 25%. The microscopic origin of these instabilities could be related to the formation of ellipsoidal Fermi surfaces centered at the L point due to the metallization of the hydride by the Mg substitution. Applying the quasiharmonic approximation, thermodynamic properties like heat capacities, vibrational entropies and vibrational free energies as a function of temperature at zero pressure are obtained. These properties determine an upper temperature for the thermodynamic stability of the hydride, which decreases from 600 K for NaH to 300 K at 20% Mg concentration. This significant reduction of the stability range indicates that dehydrogenation could be favoured by electron doping of NaH.