Letter: Fe³⁺ spin transition in CaFe₂O₄ at high pressure

Abstract

Single-crystal diffraction data collected for CaFe₂O₄ at high pressure reveal above 50 GPa an isosymmetric phase transition (i.e., no change in symmetry) marked by a volume decrease of 8.4%. X-ray emission spectroscopic data at ambient and high pressure confirm that the nature of the phase transition is related to the Fe³⁺ high-spin/low-spin transition. The bulk modulus K₀ calculated with a Birch Murnaghan EoS (K′ = 4) is remarkably different [K₀ = 159(2) GPa for CaFe₂O₄ “high spin” and K₀ = 235(10) GPa for CaFe₂O₄ “low spin”]. Crystal structure refinements reveal a decrease of 12% of the Fe³⁺ crystallographic site volume. The geometrical features of the low-spin Fe³⁺ crystallographic site at high pressure (bond lengths, volume) indicate a relevant decrease of Fe³⁺-O bond lengths, and the results are in agreement with tabulated values for crystal radii of Fe³⁺ in high- and low-spin state. The reduced crystal size of Fe³⁺ in the low-spin state suggest that in lower mantle assemblages, Fe³⁺ partitioning in crystallographic sites should be strongly affected by the iron spin state.