Fe(3+) spin transition in CaFe2O4 at high pressure

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.