Inelastic x-ray and neutron scattering were used to measure two matching lattice excitations on the $\left[01\zeta \right]$ zone boundary in $\alpha$-uranium. The excitations have the same polarization and reciprocal-space structure, but one has energy consistent with the thermal activation energy of the other, indicating that it creates the mode. The implied mechanism, where a mode is created by an amplitude fluctuation that mirrors the mode itself, is consistent with an intrinsically localized mode (ILM), and this is supported by thermodynamic data. The reciprocal-space structure, however, indicates a mode that is extended along its polarization direction, [010], and yet fully localized along a perpendicular direction, [001]. An enhancement of the thermal but not electrical conductivity with mode activation also suggests that these modes are more mobile than conventional ILMs. The behavior is, however, qualitatively similar to that predicted for ILMs on two-dimensional hexagonal lattices, where in-plane localization has been shown to be extended over more than ten discrete units, and the modes can be highly mobile.

• Received 26 May 2008

DOI:https://doi.org/10.1103/PhysRevB.77.214305