The first 30 years of research on the physical properties of α-U

Abstract

Uranium metal has three allotropic phases in the solid state, with a melting point of 1405 K. The α phase crystal (0–940 K) has orthorhombic symmetry, which is unique among unalloyed metals, and, because of its dimensional instability as a fuel element in early nuclear reactors, has a nearly unique history as a subject for solid state research. The need to understand its behavior under neutron irradiation sparked a world-wide effort to investigate the anisotropic properties of the crystal and its polycrystalline aggregates. Methods for metal purification and for preparing single crystals were followed by very precise measurements of mechanical and physical properties in general. The unusual scope of the research was enhanced by the discovery of a second-order phase transition at 43 K and speculations regarding magnetic structures and superconductivity of uranium and the other 5f elements. In this review some of the early research beginning in 1948 and the low temperature and high pressure research that followed up to 1978 are described. This precedes the surprising discovery of the charge-density-wave structures that emerged from the lattice dynamic studies and neutron elastic scattering in 1979 and 1980.