Analysis of the enthalpy of mixing data of binary and ternary [rare earth (Nd, La, Y, Yb), Al–alkali metal]–fluoride systems

Abstract We analyzed the enthalpy of mixing data of several liquid [rare earth and aluminum–alkali metal] fluoride systems using the Hoch–Arpshofen solution model. We investigated the binary systems AlF3–MF (M is Li, Na, K), (LaF3, YF3, YbF3)–MF (M is Li, Na, K, Rb, Cs) and NdF3–MF (M is Li, Na, K) and the ternary LaF3–NaF–LiF system. We also analyzed the LaBr3–MBr (M is Li, Na, K, Rb, Cs) system, to show the difference to fluoride systems. In the fluoride systems the experimental data is carried out with solid MeF3 and thus the analysis depends on the enthalpy of fusion of the MeF3 much below the normal melting point. In the fluoride systems the enthalpy of mixing can be represented by a regular solution; in other systems the enthalpy of mixing leans toward the alkali metal. There is a linear relationship between the maximum enthalpy of mixing and the radius of the alkali metal cation, and also the halide anion radius. In the ternary system LaF3–NaF–LiF the enthalpy of mixing could be computed from the binary systems. The relationship between the maximum of the enthalpy of mixing and the radii of the three valent cations, the alkali cations and the halide anions is: (a) only fluorides: H m,max (kK) = −(7.714±0.876)+(10.293±0.709)∗r M 3+ −(4.285±0.537)∗r M + R=0.9803 (b) all [Al, rare earth–alkali metal]–halides analyzed: H m,max (kK) = −(10.814±0.605)+(10.017±0.622)∗r M 3+ −(3.001±0.267)∗r M + +(1.592±0.337)∗r L − R=0.9643