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  • Author: Marcelle Gaune-Escard x
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Summary

Phase equilibria in the molten system KF-K2NbF7-Nb2O5 were investigated up to 20mol% of Nb2O5 by thermal analysis and differential scanning calorimetry. In the binary and ternary systems oxofluoroniobate compounds are formed. They were evidenced by comparison of experimetal X-ray diffraction patterns with those in the international X-ray diffraction database (PDF-2). In the experimental compostion range the phase diagram of the molten KF-K2NbF7-Nb2O5 system consists of five crystallisation fields corresponding to KF, K3NbF8, K2NbF7, K2NbO3F and K3NbOF6.

The molar enthalpies of mixing (ΔmixHm) of liquid PrCl3-LiCl and NdCl3-LiCl mixtures have been measured at 1044 K. For both systems these enthalpies are negative in the whole composition range, with a minimum at xPrCl3 and xNdCl3 ≈ 0.4. The results are compared with existing mixing enthalpy data on lanthanide chloride-alkali metal chloride systems and discussed in terms of complex formation in the melts.

The electrical conductivity of molten MCl-NdCl3 (M = Li, Na, Rb and Cs) has been measured from the liquidus temperature up to ~ 1180 K. The measurements were performed in usual U-shaped capillary quartz cells with platinum electrodes. The molar conductivity (Λ) has been computed by using literature data on the densities of the binary systems. In all cases, when the temperature range exceeds about 100 K, the plot lnΛ vs. 1/T is not a straight line. The activation energy of the conductivity does not remain constant but reduces with increasing temperature. In the specific and molar conductivity isotherms strong deviations from additivity are noted. The results are discussed in terms of octahedral local coordination of Nd3+ over the entire concentration range.

The phase equilibrium of the TbBr3-KBr has been established by Differential Scanning Calorimetry. This system has the three compounds K3TbBr6, K2TbBr5, and KTb2Br7 and two eutectics located at (χTb = 0.163 (885 K) and (χTb = 0.433 (697 K). K3TbBr6 undergoes a solid-solid phase transition at 691 K and melts congruently at 983 K with the corresponding enthalpies 8.0 and 48.0 kJ mol−1. K2TbBr5 melts incongruently at 725 K, and KTb2Br7 at 741 K. The latter forms at 694 K, a temperature very close to that (697 K) of one of the two eutectics also existing in the binary system.

Formation of oxyfluoroniobium compounds in the binary systems KF-Nb2O5 and K2NbF7-Nb2O5 and in the ternary system KF-K2NbF7-Nb2O5 has been expected. Therefore the phase equilibrium and surface tension of the above systems have been determined. The three systems have only been investigated up to 20 mol% Nb2O5 because of its limited solubility. The obtained results have confirmed the formation of oxyfluoroniobium compounds.

Abstract

Molecular dynamics simulations of molten DyCl3-NaCl were carried out at liquidus temperatures of the phase diagram. The chemical potential and the activity of NaCl was successfully estimated with the method proposed by Powles et al., which requires only positional data of the ions at the temperatures in question.

Phase equilibrium in the PrBr3-CsBr binary system was established from Differential Scanning Calorimetry (DSC). This system has two compounds, Cs3PrBr6 and CsPr2Br7, and three eutectics located at molar fraction of PrBr3 (x = 0.108; 850 K), (x = 0.453; 767 K), and (x = 0.757, 870 K), respectively. Cs3PrBr6 undergoes a solid-solid phase transition at 726 K and melts congruently at 1051 K. CsPr2Br7 undergoes a solid-solid phase transition at 835 K, and melts congruently at 896 K. The electrical conductivity of PrBr3-CsBr liquid mixtures was measured down to temperatures below solidification over the whole composition range. Results obtained are discussed in term of possible complex formation.

Differential scanning calorimetry (DSC) was used to investigate the phase equilibrium in the AgCl- NdCl3 system. This binary mixture represents a typical example of simple eutectic system, with eutectic composition x(AgCl)=0.796 and temperature Teut = 668 K, respectively. The electrical conductivity of AgCl-NdCl3 liquid mixtures, together with that of pure components was measured down to temperatures below solidification. Results obtained are discussed in terms of possible complex formation.