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 $(P\text{\ensuremath{-}}T)$ are of great importance for understanding the structural heterogeneity of Earth's mantle. Here, we investigate the phase diagram and thermoelastic properties of iron oxyhydroxide (FeOOH) across the spin crossover using dynamic high $P\text{\ensuremath{-}}T$ experiments and theoretical simulations. The Hugoniot equation of state in FeOOH has been measured up to $\ensuremath{\sim}90$ GPa and $\ensuremath{\sim}2100$ K in a two-stage light-gas gun and exhibits a density discontinuity between 47 GPa (\ensuremath{\sim}950 K) and 61 GPa (\ensuremath{\sim}1150 K) due to the high-low spin transition of ${\mathrm{Fe}}³⁺$, which is consistent with our first-principles calculations. The $P\text{\ensuremath{-}}T$ 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 \ensuremath{\sim}60% softening of adiabatic bulk modulus and a negative Poisson's ratio (\ensuremath{-}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 $P\text{\ensuremath{-}}T$ 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.