Structural relaxation in layered, non-stoichiometric Fe₇S₈

In this study, we investigate the kinetics of the enantiotropic solid-solid β-transition in Fe7S8 pyrrhotite, which presents a prominent example of a metal-nonmetal compound with layered crystal structure. The low-temperature (4C) and high-temperature (1C) modifications differ in their crystallographic unit-cell dimension, vacancy distribution, and magnetic ordering in the crystal lattice. Fast differential scanning calorimetry (FDSC) reveals that cooling of the paramagnetic 1C phase below the transformation temperature Tβ = 597 K, which is also the Curie temperature, generates a metastable phase that transforms into the ferrimagnetic 4C phase with high vacancy order upon further annealing below Tβ. Upon fast cooling, the low-temperature modification shows an energetically excited phase with higher entropy that relaxes towards the equilibrated pyrrhotite polymorph. The kinetics of the superheating and the structural relaxation as obtained from FDSC experiments provide deeper insight into the stability of Fe7S8 polymorphs. This may pave a new path to decipher in detail the kinetics of solid-solid phase transformations and the long-term lifespan of defects in Earth and synthetic materials.