ENTHALPIES OF PHASE-TRANSITION IN THE LANTHANIDE CHLORIDES LACL3, CECL3, PRCL3, NDCL3, GDCL3, DYCL3, ERCL3 AND TMCL3

The enthalpies of phase transition in the lanthanide chlorides have been measured with a differential scanning calorimeter and also with a Calvet-type microcalorimeter to within an experimental error of +/- 2%. The molar enthalpies of fusion of LaCl3, CeCl3, PrCl3, NdCl3, GdCl3, DyCl3, ErCl3 and TmCl3 are 55.7, 55.5, 52.1, 48.1, 40.6, 22.8, 31.1 and 35.6 kJ mol-1 respectively. The existence of a solid-solid phase transition was found in DyCl3 and ErCl3 at 611 and 1025 K with a molar enthalpy of transition of 1.4 and 5.3 kJ mol-1 respectively. A significant difference (8.5 kJ mol-1) between the absolute values of molar enthalpies of crystallization and fusion was found for LaCl3 at 1124 +/- 3 K; this unusual behaviour was explained by an ''after-fusion'' effect. The entropies of fusion are discussed in terms of structural types of the lanthanide chlorides.