Journal of Thermal Analysis, VoL 45 (1995) 1117-1124 CALORIMETRIC INVESTIGATIONS OF THE MBr-NdBr3 MELTS (M=Li, Na, K, Cs) Marcelle Gaune-Escard t, A. Bogacz 2, L. Rycerz 2 and 9 2 W. Szczepamak IlUSTI, UAl168 CNRS, Universit6 de Provence, Centre de St. J6r6me, av. Escadrille Normandie - Niemen, 13397 Marseille Cedex 20, France 2Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Technical University, Wybrzeze Wyspiafiskiego 27, 50-370 Wroctaw, Poland Abstract Present work is a part of thermodynamic research program on the MX-LnX3 system (M= al- kali metal, X=CI, Br and Ln=lanthanide). Molar enthalpies of mixing in the LiBr-NdBr3, NaBr-NdBr3 and KBr-NdBr3 liquid binary systems have been determined at temperature 1063 K by direct calorimetry in the whole range of composition. Investigated systems are generally char- acterized by negative enthalpies of mixing with minimum at XNdB~3~0.3--0.4. These enthalpies de- crease with decrease of ionic radii of alkali metals. Molar enthalpies of solid-solid and solid-liquid phase transitions of K3NdBr6 and Cs3NdBr6 have been also determined by differential scanning calorimetry (DSC). K3NdBr~ is formed at 689 K from KBr and K2NdBr5 with enthalpy of 44.0 kJ.moV ~whereas Cs3NdBr~ is stable at am- bient temperature and undergoes phase transition in the solid state at 731 K with enthalpy of 8.8 kJ-mol-I . Enthalpies of melting have been also determined. Keywords: alkali metal bromides, DSC high-temperature calorimetry, mixing enthalpy, neodynium bromide, phase transition Introduction Presented in this work investigations are part of general research program concerning thermodynamic properties and electrical conductivity of MX-LnX3 systems (where M=Li, Na, K, Rb, Cs; Ln=La, Ce, Pr, Nd; X=CI, Br). Spe- cial attention has been paid to solid and liquid compounds M3LnX6. They exist only in systems with M=K, Rb, Cs [1-5]. Enthalpies of mixing in systems LiBr-NdBr3, NaBr-NdBr3 and KBr-NdBr3 have been determined as well as en- thalpies of mixing in systems RbBr-NdBr3 and CsBr-NdBr3 at Xr~dBr3=0.25. Molar enthalpies and entropies of phase transitions of K3NdBr6 and Cs3NdBr6 compounds were also determined by differential scanning calorimetry. 0368-4466/95/$. 4. O0 John Wiley & Sons, Limited 9 1995 AkadOmiai Kiad6, Budapest Chichester