Structure of siderite FeCO3 to 56 GPa and hysteresis of its spin-pairing transition

The structure of siderite, FeCO3, was determined to 56 GPa, beyond the spin-pairing transition of its iron d electrons. Fe2+ in the siderite structure is in the high-spin state at low pressures and transforms to the low-spin (LS) state over a narrow pressure range, 44 to 45 GPa, that is concomitant with a shrinkage of the octahedral bond distance by 4%, and a volume collapse of 10%. The structural rearrangements associated with the electronic transition are nearly isotropic in contrast with other properties of siderite, which mostly are highly anisotropic. Robust refinements of the crystal structure from single-crystal x-ray diffraction data were performed at small pressure intervals in order to accurately evaluate the variation in the interatomic distances and to define the geometry of the carbonate hosting LS-Fe2+. Thermal vibrations are remarkably lowered in the LS-Sd as shown by atomic displacement parameters. The formation of like-spin domains at the transition shows a hysteresis of more than 3 GPa, compatible with a strong cooperative contribution of neighboring clusters to the transition.