Tetragonal BaCoO$_3$: A Co$^{4+}$ Ferromagnetic Mott Insulator with Inverted Spin Crossover

Mingyu Xu; Haozhe Wang; Krishna Prasad Koirala; Corey Melnick; Cheng Peng; Mario U. González-Rivas; Jiaqi Lu; Le Wang; Mark H. Engelhard; Yingge Du; Xianglin Ke; Robert J. Green; Alannah M. Hallas; Jie Li; Gabriel Kotliar; Weiwei Xie

arXiv · 2410.18826

Tetragonal BaCoO$_3$: A Co$^{4+}$ Ferromagnetic Mott Insulator with Inverted Spin Crossover

Mingyu Xu·Haozhe Wang·Krishna Prasad Koirala·Corey Melnick·Cheng Peng·Mario U. González-Rivas·Jiaqi Lu·Le Wang·Mark H. Engelhard·Yingge Du·Xianglin Ke·Robert J. Green·Alannah M. Hallas·Jie Li·Gabriel Kotliar·Weiwei Xie

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Abstract

The interplay between crystal electric field splitting of d states and Hund's rule exchange energy in cobalt-based perovskites offers a promising avenue for inducing spin-state transitions. This study reports a new body-centered tetragonal (BCT) phase of BaCoO$_3$ (BCT-BaCoO$_3$), synthesized under high pressure (15 GPa) and high temperature (1200 °C) conditions. BCT-BaCoO$_3$ adopts a double perovskite structure of EuTiO$_3$-type (space group I4/mcm, #140), confirmed by high-resolution scanning transmission electron microscopy. X-ray photoelectron spectroscopy reveals a rare Co$^{4+}$ valence state. Magnetization and X-ray absorption measurements reveal a low-spin to high-spin transition that takes place between 200 and 300 K. While spin crossovers are relatively common among common oxides, the one observed in BCT-BaCoO$_3$ is remarkable in that it proceeds in the opposite direction from conventional spin transitions. BCT-BaCoO$_3$ exhibits a low-spin (S = 1/2) state at high temperatures and transitions to a high-spin (S = 5/2) state at low temperatures. Within the high-spin state, hard ferromagnetic order onsets at T$_C$ = 107 K. Electrical resistivity indicates weak magnetoresistance and insulating behavior. Overall, BCT-BaCoO$_3$ presents an exceptional model for the exploration of spin-state transitions and the study of Co spin states in cobalt-based perovskites.

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Mingyu Xu, Haozhe Wang, Krishna Prasad Koirala, Corey Melnick, Cheng Peng, Mario U. González-Rivas, Jiaqi Lu, Le Wang, Mark H. Engelhard, Yingge Du, Xianglin Ke, Robert J. Green, Alannah M. Hallas, Jie Li, Gabriel Kotliar, Weiwei Xie. 2024-10-24. Tetragonal BaCoO$_3$: A Co$^{4+}$ Ferromagnetic Mott Insulator with Inverted Spin Crossover. https://arxiv.org/abs/2410.18826

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