Pressure-induced large volume collapse and possible spin transition in HP-PdF₂-type FeCl₂

Iron hydroxide FeO₂H_x (x ≤ 1) and ferrous iron chloride FeCl₂ can adopt the HP-PdF₂-type (space group: \(P{a_{\overline 3 }}\) , Z = 4) structure in the lowermost mantle, potentially contributing to the geochemical cycles of hydrogen and chlorine within Earth’s deep interior, respectively. Here we investigate the high-pressure behavior of HP-PdF₂-type FeCl₂ by X-ray diffraction (XRD) and Raman measurements in laser-heated diamond anvil cells. Our results show that HP-PdF₂-type FeCl₂ can be formed at 60‒67 GPa and 1650‒1850 K. Upon cold decompression, the diffraction peaks at pressures above 10 GPa can be indexed to the HP-PdF₂-type structure. Intriguingly, the calculated cell volumes reveal a remarkable decrease of ΔV / V = ∼ 14% between 36 and 40 GPa, which is possibly caused by a pressure-induced spin transition of Fe²⁺ (HS: high-spin → LS: low-spin). We also observe distinct changes in Raman spectra at 33‒35 GPa, practically coinciding with the onset pressures of isostructural phase transition in XRD results. Our observations combined with previous studies conducted at megabar pressures suggest that HP-PdF₂-type FeCl₂, with a wide pressure stability range, if present in subducting slabs, could facilitate the transport of chlorine from the middle lower mantle to the outer core.