Melting-Point Anomalies

Nature of the5fstates in actinide metals

“…Examining Fig. 3 shows that as the early actinide metals are traversed several changes occur near Pu: the melting temperature reaches a minimum (Matthias et al, 1967;Kmetko and Hill, 1976), the number of phases increases to a maximum, and the crystal structures become exceedingly complex (Zachariasen and Ellinger, 1963). Whereas most metals are usually cubic or hexagonal, U, Np, and Pu exhibit tetragonal, orthorhombic, and even monoclinic crystal structures, the last being an atomic geometry usually found in minerals (Klein and Hurlbut, 1993).…”

“…At early stages of actinide research, the low-symmetry crystal structures of the light actinide metals was ascribed to directional covalent bonds (Matthias et al, 1967). However, over time it became evident that the 5f states in Th-Pu are delocalized and, to varying degrees, band-like.…”

Melting of the light actinides

“…INTRODUCTION At 914K. the melting point of Pu metal is unusually low, given its position in the periodic system (Matthias et al 1967). This anomaly is coincident with other anomalous properties of the light actinides, especially the occurrence of complex crystal structures (Smith and Kmetko 1983).…”

Thermophysical properties of solid and liquid thorium

“…The role of the 5f electrons of actinides is well displayed by comparing them with rare earth (or 4f) elements whose 4f electrons are rather localized (except for Ce and Pr). One of these characteristics is the observation of a strong contraction of the atomic radius from Th to Pu [2,3] similar to the one of 3d, 4d, and 5d transition elements. This variation is correlated with very dense crystal lattices of low symmetry at ambient conditions and also the increasing number of allotropic structures (2 for Th and 6 for Pu) in the P-T phase diagram.…”