Disintegration Energy of Ferrocene Crystal in Triclinic Phase and Kinetic Study on Monotropic Transition from Monoclinic to Orthorhombic Phase

Abstract

Cooling of single crystals of ferrocene below the λ-type phase transition at 163.9 K leads to crystal disintegration with explosive violence independently of the λ-type transition. The strain energy evolved at the disintegration (“the disintegration energy” for short) was determined to be (1.10 ± 0.11) kJ mol^–1 by a drop calorimetry system using a low-temperature Calvet microcalorimeter. The disintegration energy was independent of the crystal size with four sieve fractions in the range 2.5–0.25 mm. The violent disintegration is interpreted here in terms of an energy transfer from the strain energy accumulated in the domain boundaries in a form of elastic energy to the kinetic energy of disintegrated crystallites. The speed of a crystallite with the grain size of 60–70 pm was estimated to be 110 m s ¹. On the other hand, kinetic aspect of a monotropic transition from the undercooled monoclinic to the stable orthorhombic phase at around 190 K. has been investigated with the same drop calorimetry system. The monotropic transition was found to occur after a long induction period of 1–4 h. Kinetics of the transition can be well accounted for in terms of the Avrami theory. Growth of the stable phase proceeds three-dimensionally.