Modeling and Calculation of the LiF−NaF−MF₃ (M = La, Ce, Pu) Phase Diagrams

Abstract

The ternary systems LiF-NaF-LaF 3 and LiF-NaF-CeF 3 have been experimentally analyzed and thermodynamically assessed. Two models were used and compared to describe the excess Gibbs terms of the binary subsystems. One was a regular polynomial description of the excess terms, the other a quasichemical in the quadruplet approximation. In both cases, a fine agreement with the experimental data was achieved, after asymmetrical extrapolation from the binary to the ternary systems. The binary systems of the related actinide system LiF-NaF-PuF 3 were assessed, making use of experimental data known from the literature. The lanthanide systems served as a proxy for LiF-NaF-PuF 3 to calculate the ternary phase diagram.

Calorimetric analysis of NaF and NaLaF4

“…This resulted in an excellent fit with the experimental data, but the compound is only stable above 810 K. This result is shown in figure 5, where it is compared to the old diagram, which was calculated by optimizing the complete Gibbs free energy function of NaLaF 4 [2]. The standard enthalpy of formation, the standard entropy and the heat capacity functions that were used are found in table 6.…”

“…Above this temperature, the compound is decomposed in solid LaF 3 plus a liquid, persisting in equilibrium up to 1400 K, where the liquidus is crossed [2]. The data from 1060 K to 1213 K in figure 3 cannot therefore be interpreted as the C p,m of liquid NaLaF 4 .…”

“…Platinum crucibles with a cover and with a boron nitride liner were filled with the salt mixture. The compound NaLaF 4 melts peritectically around 1060 K [1,2]. To reach an optimum mixing, the salt samples were heated above the melting temperature, up to 1100 K at a rate of 5 K AE min À1 .…”

“…The C p,m functions of NaF and LaF 3 , which were used in a previous paper [2] to assess (LiF + NaF + LaF 3 ), are added here. It can be seen that this rule is only valid in the temperature range between 300 K and 700 K. At higher temperatures the Neumann-Kopp rule considerably underestimates the observed C p,m data, probably because the lattice interactions in the compound are significantly different compared to those in the pure components, leading to different anharmonic contributions to the heat capacity.…”

“…The thermodynamic assessment is briefly described here; more details can be found in van der Meer et al [2].…”