Lanthanide(III) halides: Thermodynamic properties and their correlation with crystal structure

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Temperatures and enthalpies of phase transitions of 17 lanthanide(III) halides determined experimentally are reported. Correlations were made between temperature of fusion of lanthanide(III) halides, on the one hand, and enthalpy of fusion, on the other, versus atomic number of lanthanide. According to this classification, the lanthanide(III) halides split into groups, as also do the corresponding crystal structures. A correlation between the crystal structure of lanthanide(III) halides and their respective entropy of fusion (or entropy of fusion + entropy of solid–solid phase transition) was inferred from the aforementioned features. Fusion in those halides with hexagonal, UCl 3-type and orthorhombic, PuBr 3-type, structures entails an entropy of fusion change (or entropy of fusion + entropy of solid–solid phase transition change) by 50 ± 4 J mol −1 K −1. The homologous entropy change within the group of halides having the rhomboedric, FeCl 3-type, structure, is smaller and equals 40 ± 4 J mol −1 K −1. Halides with monoclinic, AlCl 3-type, crystal structure constitute a third group associated to an even smaller entropy change upon fusion, only 31 ± 4 J mol −1 K −1. The halides with lower entropies of fusion also have a lower S 1300 K − S 298 K indicating either a higher degree of order in the liquid or a higher entropy in the solid at room temperatures.