Physicochemical Properties of Nonvariant Compositions of Ternary NaF-LiF-LnF3 Systems (Ln = La, Nd)

This paper presents results of investigations of the physicochemical properties of nonvariant compositions of ternary NaF-LiF-LnF₃ systems (Ln = La, Nd) in crystalline-molten transition. The investigations have been carried out by differential thermal and X-ray phase analysis, high-temperature X-ray diffractometry, viscosimetry and thermal emf method.

The ternary system NaF-LiF-LaF₃ has two nonvariant points: eutectic and peritectic. The eutectic mixture of this system has the composition (mol.%): NaF(44.0)–LiF(42.0)–LaF₃(14.0) and melts at 580±2°C. The peritectic has the composition (mol.%): NaF(45.0)–LiF(39.0)–LaF₃(16.0) and melts at 595±2°C. Unlike the foregoing system, the ternary system NaF–LiF–NdF₃ has one eutectic and two peritectics. The eutectic has the composition (mol.%): NaF(33.0)–LiF(53.0)–NdF₃(14.0) and melts at 580±2°C. The peritectic, which corresponds to the formation of the compound NaNdF₄, has the composition (mol.%): NaF(41.0)–LiF(44.0)– NdF₃(15.0) and a melting point 595±2°C. The second peritectic has the composition (mol.%): NaF(39.0)–LiF(45.0)– NdF₃(16.0) and melts at 610±2°C.

An analysis of the results obtained showed that before reaching the melting temperature, the eutectic compositions of the investigated systems do not undergo structural and phase transformations. Above the melting temperature, the structure of eutectic melts is different in sprite of the very close ionic radii of rare-earth cations (r_La = 0.103 nm, r_Nd = 0.098 nm) and equal charges. The basis of the eutectic melt structure of the ternary LiF-NaF-LaF₃ system in the range 580-700 ⁰C is a fluoride matrix, formed by complex LaF₆^3- anions. The fluorine ions around the lanthanum ions form a deformed trigonal Bernal prism. The alkali metal cations are chaotically arranged in the fluoride matrix. In the LiF-NaF-NdF₃system, the fluorine ions do not form a homogeneous matrix; the melt is structurally inhomogeneous, and the neodymium cations do not form locally ordered centers with lithium and sodium polyhedra attached to them.

It has been found that the plots of thermoelectric coefficient against temperature for each of the investigated nonvariant compositions have a certain temperature T, at which their slope changes its sign from negative to positive. This temperature is about 120-160°C higher than the melting temperature of the corresponding nonvariant compositions and depends on the mode of interaction of the original reactants and the nature of the compounds that are formed in this case. The reversal of the sign of the plot of thermal emf against temperature may indicate that the nature of charge carriers changes at certain temperatures.

The structural inhomogeneity of the investigated compositions of the above ternary systems in the molten state is also evidenced by the results of viscosimetric studies (method of damped torsional vibration of cylinder). It has been found that as in the foregoing case, a certain characteristic temperature is typical of each of studied compositions, at which the slope of the temperature plot is reversed. Above this temperature, the plot of viscosity against temperature is satisfactorily approximated by the Arrhenius equation. This dependence of viscosity on temperature is likely due to dynamics of incongruent melting of the binary compounds NaLaF₄, NaNdF₄ and Na₅Nd₉F₃₂.

The identified characteristic temperatures on the plots of viscosity against temperature as well as on plots of coefficient of thermoelectric power against temperature increase in going from eutectic to peritectic compositions. Detected characteristic temperatures can be a sort quantitative measure of termination of incongruent decomposition of binary compounds NaLaF₄, NaNdF₄ and Na₅Nd₉F₃₂.