FeO (w\"ustite), which exhibits complex electronic and structural properties with increasing pressure and temperature, is a key mineralogical phase for understanding deep planetary interiors. However, direct measurements of its spin state at high-pressure and temperature remain challenging in static compression experiments. Here, we employ laser-driven shock compression to extend the FeO principal Hugoniot up to ∼900 GPa and perform in situ X-ray diffraction and X-ray emission spectroscopy up to 250 GPa, probing FeO's crystal structure and spin state. We demonstrate a continuous spin crossover of iron in FeO over a broad pressure range, with the high-spin state persisting beyond Earth's core-mantle boundary (CMB) conditions. These observations provide new experimental constraints on iron spin state at extreme conditions essential for geophysical models of (exo)planetary interiors.
@article{arxiv.2603.17136,
title = {Spin crossover in FeO under shock compression},
author = {Lélia Libon and Alessandra Ravasio and Silvia Pandolfi and Yanyao Zhang and Xuehui Wei and Jean-Alexis Hernandez and Hong Yang and Amanda J. Chen and Tommaso Vinci and Alessandra Benuzzi-Mounaix and Clemens Prescher and François Soubiran and Hae Ja Lee and Eric Galtier and Nick Czapla and Wendy L. Mao and Arianna E. Gleason and Sang Heon Shim and Roberto Alonso-Mori and Guillaume Morard},
journal= {arXiv preprint arXiv:2603.17136},
year = {2026}
}