An in situ synchrotron X-ray diffraction study was carried out on ε-FeOOH at room temperature up to a pressure of 8.6 GPa using the energy-dispersive method. The linear compressibility was determined to be βa = 1.69(3) × 10-3 GPa-1, βb = 2.86(6) × 10-3 GPa-1, and βc = 1.73(5) × 10-3 GPa-1. The b-axis of the unit cell is more compressible than the a and c axes. The pressure-volume data were fitted to a third-order Birch-Murnaghan equation of state. The best fit was found using a room temperature isothermal bulk modulus of K0 = 126(3) GPa and its pressure derivative K′ = 10(1).
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