Intrinsically localized mode in α-U as a precursor to a solid-state phase transition

The high-temperature behavior of an intrinsically localized mode (ILM) in alpha-U was measured using inelastic neutron scattering. The mode, which forms above 450 K on the [010] boundary, becomes undetectable at 675 K. Thermodynamic and transport anomalies that develop with the ILM persist to temperatures above 675 K, but mechanical and electronic anomalies show changes at both 450 and 675 K. Anisotropic thermal expansion shows that ILMs drive the structure toward hexagonal symmetry. On the [1/2 1/2 0] 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 K, causing ILMs to hop between equivalent orientations and putting the structure on a path toward the high-temperature gamma phase (bcc).