Whatʹs new on plutonium up to 4000 K?

The thermophysical properties of both solid and liquid plutonium have been investigated up to 4000 K and 0.12 GPa by use of the isobaric expansion experiment (IEX). Electrical resistivity, volume expansion (density), enthalpy, sound velocity, and equation of state parameters (bulk modulus, adiabatic and thermal compressibility, Grüneisen parameter, and the specific heat ratio) have been measured and extended in the whole liquid region up to nearly 4000 K, and are reported in this paper. Among numerous anomalies of Pu, the literature data have revealed that liquid Pu shows an atypical behavior by presenting a positive temperature coefficient for sound velocity up to 1225 K. We have confirmed such a behavior but also reported a drastic change of sound velocity at around 2000 K. Such results are interpreted in terms of the delocalization of the 5f electrons in liquid Pu.