Extended Data Fig. 4: Proton diffusion coefficients of superionic Py-FeO2H and Py-FeO2H0.5.
From: Superionic iron oxide–hydroxide in Earth’s deep mantle
a, Black, red, blue and green symbols represent proton diffusion coefficients of FeO2H at 80 GPa, FeO2H at 130 GPa, FeO2H0.5 at 80 GPa and FeO2H0.5 at 130 GPa, respectively. b, The localized superionic phase with protons distributed between two nearby O ions. c, The delocalized superionic phase with protons delocalizing from O-H-O triplet. Small grey and pink spheres denote the trajectories of the H and O ions, respectively. The intermediate phase was previously studied by Hernandez and Caracas in ice as ice phase VII”. Protons in ice VII” delocalize from the O-H-O triplet and become even more diffusive. The intermediate phase may explain the further raising of the EC in Py-FeO2Hx above ~2000 K and 121 GPa. We have been inspired to conducted FPMD simulations and have also observed the delocalized phase in Py-FeO2H at temperatures above 3000 K. We compared the trajectories of H and O ions at 2000 and 3000 K in FeO2H. At 2000 K, the protons are mostly distributed between two nearby O ions, and they diffuse in the lattice by hopping between two symmetrized hydrogen sites. At 3300 K, movement of protons no longer binds to the nearest O atoms but travels to any possible interstitial sites in the entire lattice. The greater movement of protons can be described by the delocalized superionic phase. Similar to ice VII”, this intermediate phase helps to raise the ionic conductivity from the order of 101-102 to above 103 S m−1 and becomes visible in EC experiments.
