Palladium hydride

Palladium hydride is metallic palladium that contains a substantial quantity of hydrogen within its crystal lattice. Despite its name, it is not an ionic hydride but rather an alloy of palladium with metallic hydrogen that can be written PdH_x. At room temperature, palladium hydrides may contain two crystalline phases, α and β (sometimes called α'). Pure α phase exists at x < 0.017 whereas pure β phase is realised for x > 0.58; intermediate x values correspond to α-β mixtures.^[1]

Hydrogen absorption by palladium is reversible and therefore has been investigated for hydrogen storage.^[2] Palladium electrodes have been used in some cold fusion experiments, under the hypothesis that the hydrogen could be "squeezed" between the palladium atoms to help them fuse at lower temperatures than would otherwise be required.

History[]

The absorption of hydrogen gas by palladium was first noted by T. Graham in 1866 and absorption of electrolytically produced hydrogen, where hydrogen was absorbed into a palladium cathode, was first documented in 1939.^[2] Graham produced an alloy with the composition PdH_0.75.^[3]

Making Palladium hydride[]

Metals are arranged in lattices, and in forming metallic-hydrides, the hydrogen-atoms place themselves in interstitial sites in the lattice. This is also the case for Palladium-hydride. When the surface of a Palladium lattice is brought in contact with a H₂-molecule the two hydrogens atoms split, each absorbed onto an interstitial site. The interstitial placing of hydrogen can lead to a non-stoichiometric mixture, .i.e. the ratio of Palladium and Hydrogen cannot be represented by a natural number.

The ratio in which H is absorbed on Pd is defined by ${\displaystyle x={\frac {[H]}{[Pd]}}}$ . When Pd is brought into a H₂ environment with a pressure of 1 am, the resulting concentration of H reaches x ~ 0.7. However, the concentration of H to obtain superconductivity is higher. Therefore, the concentration of H should be increased, to x > 0.75.^[4] This is done via three different routes.[footnote] It is known that hydrogen easily desorbs from palladium, therefore extra care should be taken to prevent H desorption from Pd.

The first route is loading from gas phase. A Pd-sample is placed into a high-pressure cell of H₂, in room temperature. The H₂ is added through a capillary. As a result, H is loaded onto Pd. To maintain this bonding, the pressure cell will be cooled to liquid N₂ temperature (77 K). The resulting concentration is found to be [H]/[Pd]= 0.97. ^[4]

The second route is electrochemical bonding. This is a method where the critical concentration for superconductivity can easily be exceeded without using a high-pressure environment. Via a reaction as equilibrium between H in an electrochemical phase and H in a solid phase. The hydrogen is added to Pd and Pd-Ni alloys by an H concentration of ~ 0.95.^[4] Thereafter, it has been loaded into electrolysis of 0.1n-H₂SO₄ with a current density of 50 to 150 mA/cm${\displaystyle ^{3}}$. Finally, after lowering the loading temperature to ~ 190 K, a H concentration of x ~ 1 has been reached.^[4]

The third route is known as ion implantation. Before the implantation of H ions into Pd, the Pd foil was pre-charged with H. This is done in a H₂ high temperature gas. This shortens the implantation time which follows. The concentration reached is about x ~ 0.7.^[4] afterwards the foil is cooled to a temperature of 77 K to prevent a loss of H before the implantation can take place. The implantation of H in PdH_x happens at a temperature of 4 K. The H ions penetrate in a H₂${\displaystyle ^{+}}$-beam. This results in a high concentration layer of H in a Pd foil.^[4]

Chemical structure and properties[]

Palladium is sometimes metaphorically called a "metal sponge" (not to be confused with more literal metal sponges) because it soaks up hydrogen "like a sponge soaks up water". At room temperature and atmospheric pressure (standard ambient temperature and pressure), palladium can absorb up to 900 times its own volume of hydrogen.^[5] As of 1995, hydrogen can be absorbed into the metal-hydride and then desorbed back out for thousands of cycles. Researchers look for ways to extend the useful life of palladium storage.^[6]

Size effect[]

The absorption of hydrogen produces two different phases, both of which contain palladium metal atoms in a face-centered cubic (fcc, rocksalt) lattice, which is the same structure as pure palladium metal. At low concentrations up to PdH_0.02 the palladium lattice expands slightly, from 388.9 pm to 389.5 pm. Above this concentration the second phase appears with a lattice constant of 402.5 pm. Both phases coexist until a composition of PdH_0.58 when the alpha phase disappears.^[1] Neutron diffraction studies have shown that hydrogen atoms randomly occupy the octahedral interstices in the metal lattice (in an fcc lattice there is one octahedral hole per metal atom). The limit of absorption at normal pressures is PdH_0.7, indicating that approximately 70% of the octahedral holes are occupied. When x=1 is reached, the octahedral interstices are fully occupied.^[7] The absorption of hydrogen is reversible, and hydrogen rapidly diffuses through the metal lattice. Metallic conductivity reduces as hydrogen is absorbed, until at around PdH_0.5 the solid becomes a semiconductor.^[3]

This formation of the bulk hydride does depend on the size of the catalyst Pd. When Pd becomes smaller than 2.6nm, hydrides will not be formed anymore. ^[7]

Hydrogen dissolved in the bulk differ from hydrogen dissolved on the surface. When the particles of palladium decrease in size, less hydrogen dissolves in these smaller pd particles. Therefore, relatively more hydrogen adsorbs on the surface of the small particles. This hydrogen adsorbed onto the particles do not form an hydride. Therefore, bigger particles have more places available for the formation of hydrides.^[7]

Electron and phonon band[]

The most important property of the band structure of PdH(oct) is that filled Pd states are lowered with the presence of hydrogen. Also, the lowest energy levels, which are the bonding states, of PdH are lower than that of Pd.^[8]

Additionally, empty Pd states, that are below the fermi energy, are also lowered with the presence of H.^[8]

Palladium prefers to be with hydrogen due to the interaction between the s state of hydrogen and the p states of palladium. The energy of an independent H atom lies in the energy range of the dominating p-states of the Pd bands.^[8]

Therefore, these empty states under the fermi-energy and holes in the d-band are filled.^[8]

Additionally, the hydride formation raises the fermi level above the d band. Empty states, above the d-band, are also filled. This results in filled p-states and shifts the ‘edge’ to a higher energy level.^[9]

Superconductivity[]

PdH_x is a superconductor with a transition temperature T_c of about 9 K for x = 1. (Pure palladium is not superconducting). Drops in resistivity vs. temperature curves were observed at higher temperatures (up to 273 K) in hydrogen-rich (x ~ 1), nonstoichiometric palladium hydride and interpreted as superconducting transitions.^[10][11][12] These results have been questioned^{[13][failed verification]} and have not been confirmed thus far.

A great advantage of Palladium-hydride over many other hydride-systems is that Palladium-hydride does not need to be highly pressurized to become superconducting.^[4] This makes measurements easier and gives more opportunity for different kinds of measurements (many superconducting materials require extreme pressurization to be able to superconduct, on the order of 102 GPa.^[4] Palladium-hydride could therefore also be used to explore the role that hydrogen plays in these hydride-systems being superconductors.

Susceptibility[]

One of the magnetic properties of Palladium hydride is susceptibility. The susceptibility of PdHx varies largely when changing the concentration of H.^[4] This is due to the