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Platinum Metals Rev., 1996, 40, (4), 180

The 50 K Anomaly of the Palladium-Hydrogen System

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Various explanations have been advanced to identify the origin of the 50 K anomalies of the palladium-hydrogen system, that broadly characterise discontinuous regions near 50 K in plots of the temperature dependence of various experimental parameters, including the original observation of irregularities in specific heat plots (1). There is broad acceptance of proposed correlations between the 50 K anomalies and low temperature phase modifications and associated phase diagram alterations of the palladium-hydrogen system. However, there have been puzzling absences of accompanying production of dislocation networks.

An earlier qualitative correlation of specific heat changes with ortho/para hydrogen gas inter-conversion equilibria gained little encouragement from findings of dissociated lattice hydrogen entities located in interstitial sites in the f.c.c. palladium lattice. However, by analogy with interpretations of transient bonding states in the reaction (2):

it is now suggested (1), that diffusing hydrogen atoms may be involved as specific-heat-contributing transient pairs during hydrogen dif-fusional transfers from occupied to vacant octahedral interstitial sites, as shown in the Scheme.

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References

  1. 1
    J. J. Rooney and F. A. Lewis, Pol├╝h J. Chem., 1996, 70, 662
  2. 2
    D. A. V. Kliner,, K.-D. Rinnen and R. N. Zare, J. Chem. Phys., 1989, 90, ( 8 ), 4625

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