The high-temperature behavior of an intrinsically localized mode (ILM) in $\alpha \text{−}\mathrm{U}$ was measured using inelastic neutron scattering. The mode, which forms above $450\mathrm{K}$ on the [010] boundary, becomes undetectable at $675\mathrm{K}$. Thermodynamic and transport anomalies that develop with the ILM persist to temperatures above $675\mathrm{K}$, but mechanical and electronic anomalies show changes at both 450 and $675\mathrm{K}$. Anisotropic thermal expansion shows that ILMs drive the structure toward hexagonal symmetry. On the $\left[\frac{1}{2}\frac{1}{2}0\right]$ zone boundary, which becomes equivalent to [010] under a hexagonal distortion, a normal mode shows a softening coincident with the disappearance of the ILMs. We argue that the symmetry local to the ILMs becomes hexagonal above $600\mathrm{K}$, causing ILMs to hop between equivalent orientations and putting the structure on a path toward the high-temperature $\gamma$ phase (bcc).

• Received 13 December 2007

DOI:https://doi.org/10.1103/PhysRevB.77.052301