Pressure‐Induced Phase Transitions and Electronic Properties of FeCl2: Insights from First‐Principles Calculations

The observed terrestrial depletion of heavy halogens by roughly an order of magnitude relative to their expected abundances based on volatility suggests a sequestration mechanism, possibly involving the Earth's core. The strong chemical affinity between Fe and these halogens may form iron halides within the core, potentially affecting its physical properties and longterm evolution. In this study, we employ first‐principles calculations within the generalized gradient approximation + U framework to systematically investigate the high‐pressure structural, magnetic, and electronic behaviour of FeCl 2$_2$ . The orthorhombic Pnnm$Pnnm$ phase is found to be energetically favoured up to 30 GPa. Upon compression, FeCl 2$_2$ undergoes successive phase transition from Pnnm$Pnnm$ to P3¯m1$P\bar {3}m1$ , followed by a transformation to the cubic Pa3¯$Pa\bar {3}$ phase above 42 GPa$\mathrm{GPa}$ . At high pressures, the emergence of the Pa3¯$Pa\bar {3}$ is accompanied by a magnetic collapse to the nonmagnetic state and significant volume drop. The analysis of phonon dispersion curves confirm that all identified phases are dynamically stable within their respective pressure ranges. Further inclusion of spin–orbit coupling reveals a continuous decrease in the electronic band gap with increasing pressure. The identification of a new low‐pressure phase opens a pathway for pressure‐induced property engineering in FeCl 2$_2$ and related halide systems.