Detrapping of interstitial helium in metal tritides—NMR studies

We present measurements of NMR line shapes and relaxation times for ${}_{}{}^3{}_{}{}^{}\mathrm{H}$ nuclei and their $\beta$-decay products ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ in aged samples of titanium tritide ($\mathrm{Ti}{}_{}{}^3{}_{}{}^{}\mathrm{H}_x^{}{}_{}{}^{}$). Data were obtained on samples aged 1 yr ($x=1.7\mathrm{}$) and 8 yr ($x=1.4\mathrm{}$), where $x$ is the initial concentration. These data indicate that ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ atoms are trapped interstitially in young crystals; whereas, in contrast, the older samples are found to contain helium-gas bubbles. Additionally, the younger sample was found to exhibit considerably more lattice distortion than the older sample. This—to our knowledge—is the first direct evidence for a detrapping mechanism which has been supported by a growing body of indirect evidence. We describe a theory which explains the detrapping by a percolative mechanism: When enough ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ atoms have been created to exceed a relevant critical percolation density, the helium atoms, which are considerably "oversized" for available interstices, are able to cooperatively strain the lattice beyond its elastic limit, hence irreversibly deforming the lattice. The subsequent rapid diffusion of helium would then allow heterogeneous bubble nucleation with release of helium from the crystal by blistering and/or release along grain boundaries.