Structural relaxation in layered, non-stoichiometric Fe₇S₈

Area of Science:

• Materials Science
• Solid-State Chemistry
• Geophysics

Background:

• Pyrrhotite (Fe7S8) exhibits an enantiotropic solid-solid beta-transition between low-temperature (4C) and high-temperature (1C) phases.
• These phases differ in unit-cell dimensions, vacancy distribution, and magnetic ordering.
• Fe7S8 is a metal-nonmetal compound with a layered crystal structure.

Purpose of the Study:

• To investigate the kinetics of the solid-solid beta-transition in Fe7S8 pyrrhotite.
• To understand the stability of different Fe7S8 polymorphs.
• To explore the long-term lifespan of defects in Earth and synthetic materials.

Main Methods:

• Fast differential scanning calorimetry (FDSC) was employed to study the phase transition.
• Analysis of cooling and annealing processes below the transformation temperature (Tβ).

Main Results:

• Cooling the paramagnetic 1C phase below Tβ (597 K, also the Curie temperature) forms a metastable phase.
• This metastable phase transforms into the ferrimagnetic 4C phase with high vacancy order upon annealing below Tβ.
• Fast cooling results in an energetically excited, higher-entropy low-temperature phase that relaxes over time.

Conclusions:

• The study provides insights into the kinetics of superheating and structural relaxation in Fe7S8.
• Understanding these kinetics is crucial for deciphering solid-solid phase transformations and defect behavior in materials.
• The findings contribute to understanding material stability and defect longevity in geological and synthetic contexts.