## Abstract

Utilizing simplified statistical thermodynamic treatments, analytical expressions for the pressure-composition (p-c) isotherms of hydrogen-metal systems can be derived. Fitting a set of experimental isotherms for a given hydrogen isotope, A (A = H, D or T), to the model-derived functions, two energy-related parameters can be evaluated. These parameters are the pairwise A-A nearest neighbours interaction, Η_{AA}, and an effective A-lattice interaction parameter, ε_{A}^{eff}, which includes both, the non-vibrational (electronic and elastic) contributions and the vibrational contribution. Comparing ε_{A}^{eff} for two hydrogen isotope systems (e.g. H and D) it is possible to estimate the corresponding zero-point vibrational energies of the isotopes in the hydride. This type of analysis has been performed for the Pd-H_{2}(D_{2}) system as well as for a series of the Laves phase TiCr_{2-x}Mn_{x} (X = 0, 0.5 and 1) hydrides. For each system, experimental p-c isotherms of the two hydrogen isotopes (i.e. H and D) were obtained over a wide temperature and pressure range. This included the super-critical range (i.e. above the critical temperatures of the systems) for which the model-derived p-c isotherms better represent the experimental ones. A set of microscopic energy-related parameters were evaluated for these systems. A procedure which enabled the estimation of the average zero-point vibrational energies of the isotopes in the corresponding hydride is outlined. A comparison was made between the results obtained by this procedure and corresponding reported vibrational energies measured by some vibrational spectroscopy methods. The accuracy of the model-calculated results seems to be of the order of 15-50%. A relation was obtained between the zero-point vibrational energies and the temperature where a transition occurs from a "positivé to a "negative" isotope effect.

Original language | English |
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Pages (from-to) | 88-97 |

Number of pages | 10 |

Journal | Physica Scripta |

Volume | 94 |

State | Published - 24 Oct 2001 |

Event | 5th International Workshop on Hydrogen Isotopes in Solids - Stockholm, Sweden Duration: 17 May 2000 → 19 May 2000 |

## ASJC Scopus subject areas

- Atomic and Molecular Physics, and Optics
- Mathematical Physics
- Condensed Matter Physics