Structural and electron spin state changes in an x-ray heated iron carbonate system at the Earth's lower mantle pressures

The determination of the spin state of iron-bearing compounds at high pressure and temperature is crucial for our understanding of chemical and physical properties of the deep Earth. Studies on the relationship between the coordination of iron and its electronic spin structure in iron-bearing oxides, silicates, carbonates, iron alloys, and other minerals found in the Earth's mantle and core are scarce because of the technical challenges to simultaneously probe the sample at high pressures and temperatures. We used the unique properties of a pulsed and highly brilliant x-ray free electron laser (XFEL) beam at the High Energy Density (HED) instrument of the European XFEL to x-ray heat and probe samples contained in a diamond anvil cell. We heated and probed with the same x-ray pulse train and simultaneously measured x-ray emission and x-ray diffraction of an ${\mathrm{FeCO}}_3$ sample at a pressure of 51 GPa with up to melting temperatures. We collected spin state sensitive Fe $K{\beta }_{1,3}$ fluorescence spectra and detected the sample's structural changes via diffraction, observing the inverse volume collapse across the spin transition. During x-ray heating, the carbonate transforms into orthorhombic ${\mathrm{Fe}}_4{\mathrm{C}}_3{\mathrm{O}}_{12}$ and iron oxides. Incipient melting was also observed. This approach to collect information about the electronic state and structural changes from samples contained in a diamond anvil cell at melting temperatures and above will considerably improve our understanding of the structure and dynamics of planetary and exoplanetary interiors.

Figure 1

DAC schematic with a 2.275-mm-wide standard diamond upstream and a 1.72-mm-wide Boehler-Almax diamond downstream. The sample (green) is contained in a hole in the $\mathrm{Re}$ gasket with a diameter of $100\mu \mathrm{m}$. The recorded XES signal passes out of the DAC through the standard diamond and a side window in the DAC housing (not shown). The XRD signal was measured through the downstream Boehler-Almax diamond.

Figure 2

Left: Raw emission signals for different runs (top) including the difference to a subtracted LS signal measured at $455\mathrm{kHz}$ (bottom). Center: Separated selected emission spectra for increased spin state compared to a low spin spectrum. Right: HS-reference and LS-reference of ${\mathrm{FeCO}}_3$ [9] and a combination of 44% HS spectrum and 56% LS spectrum, resembling the run at highest used pulse energy. The difference between each spectrum and the LS spectrum is shown below.

Figure 3

The dependence of $S_{\text{bulk}}$ as obtained by fitting of the $\mathrm{\Delta }$M1 shift (light blue), and by fitting of the raw XES spectra to the reference spectra (dark blue) to the pulse energy of the incident XFEL beam. The ${\mathrm{FeCO}}_3$ diffraction peak intensity ratio (red) estimates the spin state of ${\mathrm{FeCO}}_3$. The splitting of the blue and red shaded area above roughly $50\mu \mathrm{J}$ is due to the decomposition of ${\mathrm{FeCO}}_3$ and shows the effect of $\mathrm{Fe}$ bearing decomposition products on the spin state $S_{\text{bulk}}$.

Figure 4

Left: ${\mathrm{FeCO}}_3$ (012) diffraction peak measured for $455\mathrm{kHz}$ runs. At $51.9\mu \mathrm{J}$, a single LS-${\mathrm{FeCO}}_3$ peak is visible. At $78.4\mu \mathrm{J}$, two peaks are observed which can be attributed to the LS-${\mathrm{FeCO}}_3$ and the HS-${\mathrm{FeCO}}_3$. Right: ${\mathrm{FeCO}}_3$ (012) diffraction peaks measured for $2.2\mathrm{MHz}$ runs. In the XRD data at $11.2\mu \mathrm{J}$ we see a single peak of the LS phase. Afterward, the peak splits into a LS/HS set with increasing HS portion. Example detector images for cold and hot runs are given in SM Fig. 1.

Figure 5

Simulation of the heating cycle within each train at $2.2\mathrm{MHz}$ (red line) and $455\mathrm{kHz}$ (blue line). Circles mark the temperatures when the sample gets probed with XES and XRD. Phase stability fields at 51 GPa as suggested by Cerantola et al. [18] are indicated for LS-${\mathrm{FeCO}}_3$ (blue), HS-${\mathrm{FeCO}}_3$ (red), the spin state transition zone (violet), ${\mathrm{Fe}}_3{\mathrm{O}}_4$ (green), and melt (yellow).

Figure 6

Intensity-based phase distribution for the XRD (top left, see SM Fig. 3) and Raman (top right, see SM Fig. 5) maps. The main phase ${\mathrm{FeCO}}_3$ (blue) has slightly lower intensities in the heating spot where ${\mathrm{Fe}}_4{\mathrm{C}}_3{\mathrm{O}}_{12}$ (green) and ${\mathrm{CO}}_2-\mathrm{V}$ (red) appear. The XES map (bottom left, see SM Fig. 4) shows HS state in the heated spot relative to the surrounding LS-${\mathrm{FeCO}}_3$. The sample's approximate outline is marked in red. All measurements were conducted while still under high pressure. An image of the sample loaded in the DAC under a microscope is shown in the bottom right.

Figure 7

Scanning electron microscopy picture of a recovered grain (top) with melting like structures on the grain's surface. The red line shows the position where the TEM foil was cut with a focused ion beam (FIB). The bright field image of the foil (bottom) shows $\mathrm{Fe}$-oxide grains (grey) with very high porosity (white), suggesting degassing of ${\mathrm{CO}}_2$.