Heat capacity, entropy, and enthalpy of 242PuF3 from 10 to 350 °K

Abstract

Heat capacity measurements have been made on a 17.5 g sample of 242PuF3 from 10 to 350°K. The measurements from 20 to 350°K were made by the quasiadiabatic method with discontinuous electrical heating. Temperature drifts in the region 18–58°K with no electrical heating were compared with the measured heat capacities to evaluate the radioactive self-heating rate, (1.206±0.004)×10−4 W/g Pu. Heat capacities from 10 to 20°K were obtained from this radioactive self-heating rate and observations of the temperature drifts with no electrical heating. A transition was observed between 110 and 122°K, with an associated excess entropy of 0.777±0.027 J °K−1 · mole−1 and an excess enthalpy of 92.2±3.4 J mole−1. Magnetic susceptibility measurements showed that PuF3 undergoes anti-ferromagnetic ordering at 9.0±0.5°K. At 298.15°K the heat capacity Cp°, entropy S°, enthalpy H° - H0°, and Gibbs energy divided by temperature (G° - H0°)/T are, respectively, 92.64±0.28 J °K−1 · mole−1, 126.11±0.36 J °K−1 · mole−1, 18015±54 J mole−1, and −65.69±0.18 J °K−1 · mole−1. Evidence for radiation damage was observed after the compound was stored overnight in contact with a reservoir at 4.2°K, and the radiation damage annealed between 10 and 14°K.