X-Ray Absorption Fine Structure Spectroscopy of Iron Compounds at High-Energy-Density Conditions

A critical next step in understanding high-energy-density (HED) matter is to characterize both the temperature and chemistry of materials at HED conditions. Temperature measurements are historically difficult at low temperature HED conditions above 100 GPa and below 5000 K. Furthermore, when we compress matter to these extreme conditions, electron orbitals can be distorted leading to new chemistry. X-ray absorption fine structure spectroscopy is a unique technique capable of simultaneously constraining in situ both the temperature and chemistry of compressed materials. At the Omega Laser Facility, multiple iron compounds were quasi-ramp compressed to above 500 GPa and probed with a broadband x-ray source. A new x-ray spectrometer with improved spectral resolution and energy calibration was used to measure the x-ray absorption spectrum. This improved resolution allowed x-ray absorption near edge spectroscopy features of Fe₂O₃ to be measured and these data indicate continued electron orbital distortion with increasing pressure. Moreover, the modulations in the extended x-ray absorption fine structure region of the spectrum were fit using a FEFF[1]-based Bayesian inference routine to characterize the ion positions and ultimately the temperature through analytical models of the phonon spectrum and lattice potential wells.

[1] J. J. Rehr et al., Phys. Chem. Chem. Phys. 12, 5503 (2010).