American Geophysical Union (AGU) Acta Medica Okayama 0094-8276 53 10 2026 Spin Transition of Fe3+ in δ-(Al,Fe)OOH and Implication for Mid-Lower Mantle Seismic Heterogeneities e2025GL121007 EN Chaoshuai Zhao State Key Laboratory of Precision Geodesy, University of Science and Technology of China Zhu Mao State Key Laboratory of Precision Geodesy, University of Science and Technology of China Xinyue Zhang State Key Laboratory of Precision Geodesy, University of Science and Technology of China Yingxin Yu State Key Laboratory of Precision Geodesy, University of Science and Technology of China Ningyu Sun State Key Laboratory of Precision Geodesy, University of Science and Technology of China Jianbo Zhang Center for High Pressure Science and Technology Advanced Research Yuzhu Wang Shanghai Advanced Research Institute, Chinese Academy of Sciences Takayuki Ishii Institute for Planetary Materials, Okayama University δ-(Al,Fe)OOH is an important water carrier and plays a critical role on Earth's deep water cycle. Lattice parameters of δ-(Al0.89Fe0.11)OOH were measured by synchrotron single-crystal X-ray diffraction at simultaneously high temperature and pressure up to 65 GPa and 800 K in diamond anvil cells. The results reveal that the spin crossover increases from 30 to 37 GPa at 300 K to 36–48 GPa at 700 K. Moreover, at the spin crossover, the KT and VΦ of δ-(Al0.89Fe0.11)OOH occur significant elastic softening, with maximum reductions of 50% on KT and 29% on VΦ at 33 GPa and 300 K to 37% on KT and 23% on VΦ at 41 GPa and 700 K. The anomalous elastic properties of δ-(Al,Fe)OOH at the spin crossover enhance our understanding of local seismic observations anomalies and help identify potential water-rich regions in the mid-lower mantle. No potential conflict of interest relevant to this article was reported.