Structure of siderite ${\text{FeCO}}_3$ to 56 GPa and hysteresis of its spin-pairing transition

The structure of siderite, ${\text{FeCO}}_3$, was determined to 56 GPa, beyond the spin-pairing transition of its iron $d$ electrons. ${\text{Fe}}^{2+}$ 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 ${\text{LS-Fe}}^{2+}$. 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.

Figure 1

(Color online) The sample chamber at the center of the cupped anvils shrank to a diameter of $\sim 60\mu \text{m}$ at 55 GPa. Below the crystal, a cleavage rhombohedron, is a ruby sphere while the irregular dark material is fine gold powder. Crystal and pressure gauges are embedded in Ne.

Figure 2

(Color online) Unit-cell volume of siderite. Diamonds: HS-Sd, compression; circles: LS-Sd; squares: crossover, compression; solid triangles: crossover, decompression; empty triangles: HS-Sd decompression; empty circle: value from the literature (Ref. 20).

Figure 3

(Color online) Ratio of cell parameters as a function of (a) pressure and (b) their relative variation.

Figure 4

(Color online) Oxygen fractional coordinate as a function of pressure. Diamond: HS-Sd; circles: LS-Sd; squares: crossover; triangles: results from stations 16IDB and 13IDD; empty circle: literature value (Ref. 20).

Figure 5

(Color online) Refined isotropic displacement parameters of (a) iron and (b) oxygen as a function of pressure.

Figure 6

(Color online) Peak shape at different pressures, (a): 43.86 GPa, (b): 44.14 GPa, (c): 45.13 GPa, (d): 46.40 GPa, and (e) comparison of the profile of a peak before, after and at the spin transition.

Figure 7

(Color online) Compressibility of the (a) carbon-oxygen and of the (b) iron-oxygen bonds.

Figure 8

(Color online) (a) Oxygen-oxygen distances, (b) distortion of the Fe coordination site as a function of pressure and (c) nonpolyhedral oxygen-oxygen distance as a function of volume. In the inset in (a), the atomic separations are color coded as in the plot: circles: ${\text{O-O}}_{\text{L}}$, diamonds: ${\text{O-O}}_{\parallel }$, triangles: ${\text{O-O}}_{\text{T}}$.