Lattice deformation in ${\mathrm{TaT}}_{\mathrm{x}}$ caused by transmutation-produced helium is investigated by neutron-scattering techniques. The evolution of the position and width of Debye-Scherrer lines is followed in two polycrystalline samples over a period of more than two years. The observed relative lattice parameter expansion with helium concentration d(Δa/a)/${\mathrm{dc}}_{\mathrm{He}=0.10\mathrm{}}$ yields d(Δa/a)/${\mathrm{dc}}_{\mathrm{He}=0.15\mathrm{}}$ if corrected for tritium decay. Broadening of Debye-Scherrer lines indicates the presence of an evolving dislocation network. In addition, some long-wavelength phonons are investigated in a ${\mathrm{TaT}}_{0.05}$ single crystal. Comparison with a single crystal of pure Ta taken as reference shows that the phonon intensities in the tritium-charged sample are considerably reduced, however, without any noticeable change in energy and linewidth. Technologically significant, macroscopic fracture of both polycrystalline samples occurs already at helium concentrations below 1 at. %.

• Received 7 February 1986

DOI:https://doi.org/10.1103/PhysRevB.34.4985