The diffusion of these isotopes upon the (110) plane was studied by using field emission fluctuation methods. Thermally activated diffusion was found at temperatures greater than 130 to 160K. This depended upon the isotope type and upon the degree of coverage. The activation energy for H was between 4 and 5.3kcal/mol, and increased with increasing degree of coverage. Higher values were always found for D. At lower temperatures, tunnelling suddenly began to occur in the case of H. The change was more gradual in the case of D. The tunnelling diffusivities ranged from 4 x 10-14 to 10-12cm2/s for H; depending upon the degree of coverage. The corresponding limits for D were 1.2 x 10-13 and 3.6 x 10-13cm2/s, respectively. The small differences between the results for the 2 isotopes suggested that the tunnelling barriers were quite narrow. It was proposed that this corresponded to the valley between adjacent W atoms. It was also proposed that the binding site was a long narrow cell which consisted of the hour-glass shaped region between the nearest W atoms. In the thermally activated regime, the diffusivity of D increased by more than 6 orders of magnitude as the degree of coverage increased from 0.1 to 0.9. The change was less marked in the case of H. It was concluded that chemisorbed D atoms behaved in a classical manner, while chemisorbed H atoms behaved like fermions' leading to longer mean free paths for D.Diffusion of Hydrogen and Deuterium on the (110) Plane of Tungsten. R.DiFoggio, R.Gomer: Physical Review B, 1982, 25[6], 3490-511

Table 193

Diffusion of D and H on the (001) Plane of W

at Temperatures of between 220 and 350K