X-ray Diffraction, Lattice Structure, and Equation of State of PdH_xand PdD_xto Megabar Pressures

Abstract

High-pressure X-ray diffraction of PdH x and PdD x demonstrates that these materials remain in a face-centered cubic (fcc, Fm3̅m) structure to megabar pressures at room temperature. The volumes indicate stoichiometric compositions under pressure with x = 1 for both materials. No indication of any phase transition was observed up to the highest pressures reached in the experiments. A third-order Birch–Murnaghan equation of state used to fit the pressure–volume data gives V 0 = 10.73 (±0.03) cm3/mol, K 0 = 147 (±11) GPa, and K 0′ = 4.7 (±0.5), whereas a Vinet fit gives V 0 = 10.74 (±0.03) cm3/mol, K 0 = 143 (±11) GPa, and K 0′ = 5.1 (±0.5), for the combined data for both PdH x and PdD x . The results are used to obtain the pressure dependence of the effective volume of H and D atoms in PdH x and PdD x to megabar pressures for comparison with other simple hydrides with implications for superconductivity in this class of materials.

An x-ray study of palladium hydrides up to 100 GPa: Synthesis and isotopic effects

“…The volume per H atom is slightly bigger in the case of PdH. Two previous measurements of the volume of PdH under pressure give experimental values of the volume expansion per H atom bounding the present data, the determination of Brownsberger 23 above and the one of Kuzovnikov below 22 . The calculated volume expansion per hydrogen atom under pressure is in good agreement with the present determination from 3 GPa to 100 GPa.…”

“…This result is quantitatively in agreement with a similar measurements performed on lithium hydride and deuteride 36 . This is at odds with the findings of Brownsberger 23 , reporting a crossing between the PdH and PdD compression curves around 100 GPa. The need for small error bars on the data points to disclose the isotopic shift is illustrated in Fig.…”

“…Previous determinations from ref. 23 and ref. 22 are plotted as blue dashed line and green triangles respectively.…”

“…For Pd-rich alloys, it was suspected that a fraction of H atoms could also occupy (T) sites, that could lead to extra solubility of H in Pd and a H:P ratio over 1 above 20 GPa 22 . PdH was observed to remain the stable phase in a recent compression experiment of Pd in hydrogen to 100 GPa 23 . However, in this experiment, no laser-heating of the sample was coupled to high pressure so as to overcome a possible kinetic barrier for the diffusion of H atoms in (T) sites.…”

Materials advances result from study of cold fusion

“…He suspects that the hydrogen loading of palladium might be increased by pressure. Hemley and his co-workers have already seen the 1:1 stoichiometric hydride for both hydrogen and deuterium at pressures of up to 100 GPa, 6 but have so far been unable to produce any more extensively hydrided stoichiometric phases. Hemley speculates that electrochemistry could augment the effects of high pressure in efforts to attain them.…”