Absorption and diffusion of hydrogen and deuterium in tantalum at low temperatures

Abstract
The kinetics of absorption of hydrogen and deuterium into a tantalum wire have been investigated in the temperature range 78-230 K. At pressures greater than about 0.1 Pa the rate of absorption is controlled by diffusion into the metal and the amount of hydrogen or deuterium absorbed is proportional to the square root of time. Diffusion is thought to occur mainly in the β phase, which is formed just beneath the surface of the filament. Very large isotope effects are observed at low temperatures, the ratio of the diffusion coefficients for hydrogen and deuterium being approximately 220 at a temperature of 78 K. This ratio diminishes as the temperature is increased an d above 160 K it is constant at a value of 3.5. The Arrhenius plots for diffusion of hydrogen and deuterium both show a brupt changes in slope. The activation energy for diffusion of hydrogen is 8.1 kj mol-1at temperatures below 110 K, 14.1 kJ mol-1between 110 an d 160 K , and 20.3 kJ mol-1at higher temperatures. For deuterium the activation energies are 12.7 kJ mol-1below 118 K and 20.2 kJ mol-1above this temperature. The limiting jump frequencies calculated for diffusion at low temperature are anomalously small and this is thought to indicate that tunnelling occurs between adjacent sites. A proton (or deuteron) band model of the solid is used and a rough calculation is made of the frequency of tunnelling in the various bands.