Marcelle Gaune-Escard

Abstract:

All the NdBr₃-MBr binary systems (M = Li, Na, K, Rb, Cs) are characterized by negative enthalpies of mixing [1]. The minimum of molar mixing enthalpy is shifted towards the alkali bromide-rich composition and located in the vicinity of x(NdBr₃) of about 0.3-0.4. Ionic radius of the alkali metal influences the magnitude of mixing enthalpy as well as the minimum position. The smaller the alkali metal ionic radius, the smaller the absolute value of mixing enthalpy and the minimum more shifted towards the alkali bromide-rich composition. Comparison with other LnX₃-MX binary systems (Ln = lanthanide, X = Cl, Br, I) showed that mixing enthalpy depends also on lanthanide and halide ionic radii. Its absolute value increases with decrease of lanthanide ionic radius and decreases with increase of halide ionic radius. In all the NdBr₃-MBr binary systems, the value of interaction parameter λ, which represents energetic asymmetry of the melts under investigation, is negative. Its absolute value increases significantly with ionic radius of alkali metal cation. All the systems show more negative values of the interaction parameter in the alkali halide-rich compositions. Starting from potassium bromide a broad minimum appears to exist at a molar fraction of neodymium bromide x(NdBr₃) of about 0.2-0.3. This minimum can be undoubtedly ascribed to the formation of NdBr₆^3- octahedral complexes in the systems under investigation. Conclusion concerning octahedral complexes formation in investigated melts is confirmed by the results of electrical conductivity measurements of NdBr₃-MBr liquid mixtures.
Temperatures and molar enthalpies of phase transitions of all the M₃NdBr₆ congruently melting compounds (M = K, Rb, Cs) were determined and compared with data obtained for analogous chloride and bromide compounds of other lanthanides [2]. This comparison showed that M₃NdBr₆ compounds could be divided into two groups: compounds, which are formed at higher temperatures from M₂NdBr₅ and MBr, and compounds, which are stable or metastable at ambient temperature. Moreover, compounds formed at higher temperatures can exist at ambient temperature as metastable phases when cooled with high rate. On subsequent heating thermograms exothermic effect related to the decomposition of “undercooled” decomposition occurs abruptly. The heat capacities of M₃NdBr₆ compounds were fitted by equations, which provides a satisfactory representation up to temperature of the Cp discontinuity [3]. Electrical conductivity of solid phase of M₃NdBr₆ compounds correlates well with their heat capacity [3]. Specific behavior of the heat capacity and electrical conductivity dependence on temperature of solid M₃NdBr₆ compounds is undoubtedly connected with disordering of cationic sublattice formed by alkali metal cations.

M. Gaune-Escard, A. Bogacz, L. Rycerz, W. Szczepaniak, Formation enthalpies of the MBr - NdBr3 liquid mixtures (M = Li, Na, K, Rb, Cs), Thermochim. Acta, 279 (1996) 11-25
L.Rycerz, M.Gaune-Escard, Enthalpy of phase transitions and heat capacity of compounds formed in the NdBr3 - MBr systems (M = K,Rb,Cs), In "Progress in Molten Salt Chemistry 1, Prof.N.J.Bjerrum Special Volume",R.W.Berg,H.A.Hjuler Editors,Elsevier (2000), 461-465
L. Rycerz, Thermochemistry of lanthanide halides and their systems with alkali metal halides, Beau Bassin, LAP LAMBERT Academic Publishing, 2017