Structure and phase transitions of metastable hexagonal uranium thin films

We report a simple technique for the synthesis of uniaxially textured, metastable hexagonal close-packed-like uranium thin films with thicknesses between 175 and 2800 angstrom. The initial structure and texture of the layers have been studied via x-ray diffraction and reflectivity, and the time-dependent transitions of the samples into various orientations of orthorhombic alpha-U have been mapped by similar techniques. The final crystallographic orientations of each system and the timescales on which the transitions occur are found to depend on the lattice parameters of the original layer. The absence of the alpha-U (001) orientation in the transition products suggests that the transitions in these layers are mediated by mechanisms other than the [110] transverse acoustic phonon mode previously suggested for the cubic gamma-U(110) to hcp-U(00.1) to alpha-U(001) displacive phase transition. Alternative transition pathways are discussed.