Kavli Affiliate: Cheng Peng
| First 5 Authors: Mingyu Xu, Haozhe Wang, Krishna Prasad Koirala, Corey Melnick, Cheng Peng
| Summary:
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 {deg}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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