Dynamic stability of palladium hydride: An ab initio study

Abstract

We present results of our ab initio studies of electronic and dynamic properties of ideal palladium hydride PdH and its vacancy ordered defect phase Pd 3 VacH 4 with L1 2 crystal structure proposed theoretically and found experimentally. Quantum and electronic properties of these hydrides, such as phonon dispersion relations and the vacancy formation enthalpies have been studied. Dynamic stability of the defect phase Pd 3 VacH 4 with respect to different site occupation of hydrogen atoms at the equilibrium state and under pressure was analyzed. It was shown that positions of hydrogen atoms in the defect phase strongly affect its stability and may be a reason for further phase transitions in the defect phase.

Suppression of hysteresis in absorption of hydrogen by a Pd-Au alloy

“…In agreement with experiment [9], the theory indicates that the octahedral (O) absorption sites are energetically more favorable than the tetrahedral (T) sites at H/Pd ratios up to 1 [10][11][12][13]. In addition, the theory was used to clarify the specifics of the band structure [12] and phonon dispersion of PdH [12,14,15], the dynamics of H diffusion [16], the effect of lattice strain on H diffusion [11], the energetics of subsurface H absorption [17,18], and the effect of vacancies on H absorption [14,19]. (ii) For pure Au, DFT is in favor of H absorption on the T sites, whereas the H location in the O sites is predicted to be unstable [13,19].…”

Electronic structure and crystal phase stability of palladium hydrides

“…In addi-tion, the heat of formation was calculated as −0.54 eV, −0.22 eV, and +0.10 eV, respectively, all per formula unit. These values should be compared to the heats of formation of PdH calculated from these three schemes, which amount to −0.51 eV, −0.36 eV, and −0.16 eV, respectively (the LDA value for PdH compares nicely with the value of −0.53 eV reported by Isaeva et al 33 ). Hence, within the GGA schemes, PdH is more stable than PdH 2 , which is even on the verge of forming a thermodynamically stable compound.…”

“…Due to the high interest in the palladium-hydrogen system, a number of theoretical studies have been performed ranging from simplified non-self-consistent calculations using atomic potentials 20,21,25,26 to fully selfconsistent relativistic state-of-the-art electronic structure calculations as based on density functional theory and (semi-)local approximations to the exchange-correlation functional [27][28][29][30][31][32][33] . Nevertheless, even the early work by Switendick revealed considerable distortions of the Pd band structure on hydrogenation and allowed to rule out previous rigid-band considerations 20,21 .…”

“…These findings were lateron confirmed by more refined calculations [25][26][27]29 . More recent theoretical work centered about the issue of the hydrogen site in PdH, which was motivated by the afore mentioned disputes [31][32][33] .…”

Stability of a hydrogen molecule in a vacancy of palladium hydrides

“…It was claimed that the vacancy concentration can reach as high as 16% in the state of so-called superabundant vacancies [8]. Recent first-principles studies of the interaction between vacancy and hydrogen atoms support experimental indication of the appearance of superabundant vacancy complexes in hydrogenated palladium [7,9]. In this paper, we used the total energy evaluations in density functional theory as well as ab initio molecular dynamics to determine the energetics and equilibrium HeH separation for the H 2 molecule in a vacancy of palladium and palladium hydrides at a variety of hydrogen loading ratios.…”