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

Abstract

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 Na Mg 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.

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

“…The volume decreases in both cases, as the Sr-content increases, meanwhile the volume difference between the two schemes at each concentration gets larger. As discussed in a previous work for other members of the alkali-metal hydrides [15,41], the contraction of the unitary cell can also be attributed to extra-charge redistribution in the interstitial zone as the Sr-content augments in the solid solution.…”

“…It can be seen from Figure 4 that the optical region phonon spectrum softens as x increases, whereas the acoustical part remains rather unchanged with only slight increments in its frequencies. This can be related to the enhanced metallicity in the solid solution, since the electronic density redistribution leads to an increased screening that reduces the hydrogen vibrational frequencies [41]. Another feature, as the Sr-content augments, is the emergence of phonon anomalies in the optical branches at specific regions in the Brillouin zone: Γ − K, Γ − L, and in a minor degree at approximately half-way of the L − X, W − L and L − U high-simmetry paths.…”

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

“…The volume decreases in both cases, as the Sr-content increases, meanwhile the volume difference between the two schemes at each concentration gets larger. As discussed in a previous work for other members of the alkali-metal hydrides [15,41], the contraction of the unitary cell can also be attributed to extra-charge redistribution in the interstitial zone as the Srcontent augments in the solid solution.…”

“…It can be seen from figure 4 that the optical region phonon spectrum softens as x increases, whereas the acoustical part remains rather unchanged with only increments in its frequencies. This can be related to the enhanced metallicity in the solid solution, since density redistribution leads to an increased screening that reduces the hydrogen vibrational frequencies [41]. Another feature, as the Sr-content augments, is the emergence of phonon anomalies in the optical branches at specific regions in the BZ: Γ − K, Γ − L, and in a minor degree at approximately half-way of the L − X, W − L and L − U high-symmetry paths.…”

Metal Atoms (Li, Na, and K) Tuning the Configuration of Pyrrole for the Selective Recognition of C60

“…The number of charge transfer is likely related to the distinguished E bd and E int . Based on the geometries of M-based pyrrole compounds, the Na atom leans to bind H and likely forms the generally stable NaH. − …”