Phase diagram and thermoelastic property of iron oxyhydroxide across the spin crossover under extreme conditions

Spin transition and its effect on the physical properties of iron-bearing minerals at high pressure-temperature (P -T ) are of great importance for understanding the structural heterogeneity of Earth's mantle. Here, we inves-tigate the phase diagram and thermoelastic properties of iron oxyhydroxide (FeOOH) across the spin crossover using dynamic high P -T experiments and theoretical simulations. The Hugoniot equation of state in FeOOH has been measured up to -90 GPa and -2100 K in a two-stage light-gas gun and exhibits a density discontinuity between 47 GPa (-950 K) and 61 GPa (-1150 K) due to the high-low spin transition of Fe3+, which is consistent with our first-principles calculations. The P -T phase diagram indicates that the shock-elevated temperature shifts the spin transition to a higher pressure and broadens the pressure range of mixed spins. The large volume collapse of FeOOH during its spin crossover leads to remarkable elastic anomalies, with -60% softening of adiabatic bulk modulus and a negative Poisson's ratio (-0.1) of abnormal auxeticity in the mixed-spin phase. Our results suggest that FeOOH undergoes an unselective spin transition of ferric iron at the corresponding P -T conditions of the Earth's 1400-1800 km depth and exhibits drastic softening in sound velocities and elastic modulus which may be detected as seismic heterogeneities in subducting slabs of the lower mantle.