Determination of phase boundaries and diffusion parameters in tantalum hydrides in pulsed NMR

Proton spin-lattice relaxation times T/sub 1/ were measured over a wide range of temperature (77 K to 470 K) and compositions (H/Ta = 0.155 to 0.677) in the tantalum-hydrogen system at a frequency of 40,000 MHz. In the high temperature solid solution ..cap alpha.. phase, the activation energy for hydrogen diffusion was found to be 0.140 +- 0.002 eV/atom, and the value of the jump rate (or its corresponding correlation time) was found to be essentially constant throughout the range of compositions studied. The conduction electron contribution to T/sub 1/ measured in the ..cap alpha.. phase agreed qualitatively with the trend shown by previously published susceptibility data. The single phase epsilon region and the ..cap alpha.. + epsilon two-phase region were particularly noted. It could also be concluded from the measurements that the hydrogen jump rate decreased by a factor of approximately 7.2 from the ..cap alpha.. phase to the ordered phases at low temperatures and slightly decreased further in the epsilon phase. Anomalous relaxation times were found in the low temperature range (77 K less than or equal to T less than or equal to K). In this region, T/sub 1/ remains essentially constant, and does not follow the usual temperature dependence for either motional or electronic relaxation. Two possible explanations for this behavior were considered. The first involves proton cross-relaxation to the /sup 181/Ta nuclei which would sample the spectral density of magnetic fluctuations in the sample at several frequencies because of the probable very strong /sup 181/Ta quadrupole interaction strength. The second explanation postulates that the hydrogen diffusional jump path involves an intermediate metastable state