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Lanthanides & Actinides
Lanthanide Hydrides

Preparation: Heat at 300-350°C, Ln + H2 Æ LnH2

Properties of LnH2

  • black, reactive, highly conducting
  • fluorite structure
  • most thermodynamically stable of all binary metal hydrides
  • formulated as Ln3+(H-)2(e-) with e- delocalized in a metallic conduction band
  • further H can often be accommodated in interstitial sites
    • Þ frequently non-stoichiometric
      • e.g. LuHx where x = 1.83-2.23 & 2.78-3.00
  • high pressure of H2 Æ LnH3
    • reduced conductivity: salt-like Ln3+(H-)3

      except for Eu and Yb (the most stable LnII)

The Hydrogen Storage Problem
see e.g. C.N.R. Rao & J. Gopalkrishnan, New Directions in Solid State Chemistry, CUP, 1986 p. 399-405

K. Kosuge, Chemistry of Non-Stoichiometric Compounds, OUP, 1994 p. 219-230

• The use of H2 as a fuel is most attractive

Problem: Difficult to store/transport as a liquid

  • low bpt & low density
  • H2 forms explosive mixtures with air Þ explosion risk on storage!

Solution: Store hydrogen as a solid compound (hydride) from which it can be re-extracted

  • Metals show two common types of hydride-forming behaviour:-
    • Absorb Hydrogen reversibly but in small amounts

      e.g. Pd, V, Nb, Ta

    • Absorb large quantities of hydrogen, but, essentially irreversibly

      e.g. rare earths, alkaline earths, Ti, Zr

  • Intermetallic Alloys between the two classes can be useful for hydrogen storage
    • e.g. LnNi5 class of alloys

Possible Applications of Rare Earth Intermetallic Hydrides

1. Production of ultrapure hydrogen

2. Isotope Separation of deuterium and hydrogen

3. Source of fuel for motor vehicles

4. Electrodes in Protonic Batteries/Fuel Cells

5. Load Levelling in Power Stations

6. Chemical heat-pump systems

7. Useful hydrogenation agents in organic chemistry  


  • crystallize in the CaCu5 structure
  • provides 9 interstitial sites for H
  • adsorption-desorption of hydrogen occurs topotactically
    • without drastic change in structure
    • however, lattice expands by ca. 25%


  • At a certain pressure, uptake of H2 begins
  • A large amount of H2 is adsorbed at nearly constant equilibrium pressure ~ the Plateau Pressure
  • When the stoichiometry reaches LaNi5H6 further increase in pressure yields very little extra hydrogen-adsorption {at highest pressures LaNi5H8.35 is characterized}
  • Plateau Pressure ca. 2.5 atm at 298 K
    • (ideal case ca. 1 atm!)
  • Desorption is endothermic Þ faster at ca. 140°C
  • Different LnNi5 changes plateau pressures

    ~ log plateau pressure correlates linearly with unit cell volume of the LnNi5 phase

    • Þ mixtures allow tuning of equilibrium pressure Æ `designer' compounds

Dependence of Hydrogen Plateau Pressure on

Unit Cell Volume for LaNi5 compounds

{open circles LnCo5, closed circles LnNi5, open triangles LaCo5-xNix}

--Info & DownloadsBibliography  [textbook & online resources]

Source: Dr. S.J. Heyes; University of Oxford
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