Tetragonal BaCoO ₃ : A Co ⁴⁺ Ferromagnetic Mott Insulator

Area of Science:

• Materials Science
• Solid State Physics
• Crystallography

Background:

• Barium cobalt oxide (BaCoO3) is known to exhibit various phases under different conditions.
• Stabilizing metastable phases of complex oxides is crucial for discovering new materials with unique properties.

Purpose of the Study:

• To stabilize the metastable body-centered tetragonal (BCT) phase of BaCoO3.
• To characterize the structural, electronic, and magnetic properties of the stabilized BCT-BaCoO3 phase.
• To explore methods for preserving high-pressure phases at ambient conditions.

Main Methods:

• High-pressure (15 GPa) and high-temperature (1200 °C) synthesis using a mixture precursor.
• Powder X-ray diffraction and high-resolution STEM for structural analysis.
• X-ray photoelectron spectroscopy, magnetization, heat capacity, and resistivity measurements for property characterization.
• Density Functional Theory (DFT) and DFT + Dynamical Mean-Field Theory (DMFT) calculations.

Main Results:

• Stabilization of the BCT-BaCoO3 phase with EuTiO3-type structure (space group I4/mcm).
• Predominant Co4+ oxidation state confirmed, with no detectable oxygen vacancies.
• Ferromagnetic ordering observed at TC ~ 107 K, characteristic of the BCT-BaCoO3 phase.
• Insulating behavior with weak magnetoresistance, attributed to an orbitally selective transition.
• The metastable phase could not be retained in pure form at ambient pressure.

Conclusions:

• The metastable BCT-BaCoO3 phase can be stabilized under specific high-pressure/high-temperature conditions.
• Embedding the BCT-BaCoO3 phase within a disordered mixture allows for its stabilization at ambient pressure.
• This approach provides a potential route for discovering and preserving other high-pressure phases.