Phase transitions, resistivity, and the suppression of direct hydrogen-atom interactions in tantalum hydride films

Abstract

Phase equilibrium between the interstitial solution α and stoichiometric hydride β or ε phases was investigated in tantalum hydride films, 5 μm in thickness, on Al 2O 3 ceramic substrates, by determination of the phase transition temperatures in samples with various fixed proportions of hydrogen. This was the first study of canonical phase transitions in metal hydride films. The transition temperatures were determined by accurate resistivity measurements. The experimental results were interpreted using a new phase equilibrium condition which incorporates the effect of the existence of the hydride phase on the number of available interestitial sites in the solution phase. There are significant differences observed between the film and bulk systems. The temperature-dependent part of the resistivity is greater in the films. The film phase transition data fit the new equilibrium condition almost exactly, indicating that the direct hydrogen-atom interactions in the α phase are completely suppressed. No isotopic effects were observed. The hydrogen-atom site energy difference between the two phases is 0.101 eV in the films, compared with 0.112 eV in the bulk.