ENTHALPY INCREMENT MEASUREMENTS OF PBI2 - EVIDENCE FOR A REVERSIBLE POLYTYPIC TRANSITION

The enthalpy increment of solid and liquid PbI2 have been measured by drop calorimetry from 528 to 763 K and the X-ray diffraction pattern h as been recorded from 300 to 633 K. The calorimetric data show evidenc e for a reversible transition at similar to 580 K, the enthalpy differ ence between the two phases being small, 339 J mol(-1). The X-ray diff raction results show a transition of the 2H to the 4H polytype at simi lar to 440 K, for which the reverse reaction is not observed. The melt ing point of PbI2 has been found at (679 +/- 1) K by DSC, the enthalpy of fusion was obtained as Delta(fus)H degrees = 23471 J mol(-1) from the enthalpy increments of the solid and liquid phase.