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 BaCoO3 (BCT-BaCoO3), synthesized under high pressure (15 GPa) and high temperature (1200 {\deg}C) conditions. BCT-BaCoO3 adopts a double perovskite structure of EuTiO3-type (space group I4/mcm, #140), confirmed by high-resolution scanning transmission electron microscopy. X-ray photoelectron spectroscopy reveals a rare Co4+ 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-BaCoO3 is remarkable in that it proceeds in the opposite direction from conventional spin transitions. BCT-BaCoO3 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 TC = 107 K. Electrical resistivity indicates weak magnetoresistance and insulating behavior. Overall, BCT-BaCoO3 presents an exceptional model for the exploration of spin-state transitions and the study of Co spin states in cobalt-based perovskites.
@article{arxiv.2410.18826,
title = {Tetragonal BaCoO$_3$: A Co$^{4+}$ Ferromagnetic Mott Insulator with Inverted Spin Crossover},
author = {Mingyu Xu and Haozhe Wang and Krishna Prasad Koirala and Corey Melnick and Cheng Peng and Mario U. González-Rivas and Jiaqi Lu and Le Wang and Mark H. Engelhard and Yingge Du and Xianglin Ke and Robert J. Green and Alannah M. Hallas and Jie Li and Gabriel Kotliar and Weiwei Xie},
journal= {arXiv preprint arXiv:2410.18826},
year = {2024}
}