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 has been recorded from 300 to 633 K. The calorimetric data show evidence for a reversible transition at 580 K, the enthalpy difference between the two phases being small, 339 J mol^?1. The X-ray diffraction results show a transition of the 2H to the 4H polytype at 440 K, for which the reverse reaction is not observed. The melting point of PbI₂ has been found at (679 ± 1) K by DSC, the enthalpy of fusion was obtained as Δ_fusH° = 23471 J mol^?1 from the enthalpy increments of the solid and liquid phase.