Trigonal-Planar Low-Spin Co²⁺ in a Layered Mixed-Polyhedral Network from Topotactic Reduction

Area of Science:

• Solid-state chemistry
• Materials science
• Crystallography

Background:

• Perovskite oxides are crucial in advanced materials due to their tunable properties.
• LnBaCo₂O₅₊δ compounds exhibit complex magnetic and conductive behaviors influenced by oxygen stoichiometry.
• Understanding structure-property relationships is key for designing new functional materials.

Purpose of the Study:

• To synthesize and characterize a new crystalline phase of TbBaCo₂O₅₊δ via topotactic reduction.
• To investigate the structural, magnetic, and conductive properties of the resulting TbBaCo₂O₄.₅ phase.
• To explore the role of anion vacancy ordering in influencing the material's properties.

Main Methods:

• Topotactic reduction using calcium hydride (CaH₂).
• X-ray diffraction for structural analysis and phase identification.
• Magnetic susceptibility measurements.
• Variable-temperature neutron diffraction for magnetic structure determination.
• Electrical conductivity measurements.

Main Results:

• A new phase, TbBaCo₂O₄.₅, with a 2 × 2 × 1 superstructure was successfully synthesized.
• The crystal structure features corner-shared CoO₅ square pyramids and CoO₃ trigonal planar units.
• Antiferromagnetic ordering of G-type was observed below T<0xE2><0x82><0x99> ≈ 322 K, with distinct low-spin and high-spin Co²⁺ states.
• TbBaCo₂O₄.₅ exhibits lower p-type conductivity compared to TbBaCo₂O₅.₅.

Conclusions:

• Topotactic reduction provides a route to new perovskite phases with controlled oxygen stoichiometry.
• The unique arrangement of oxygen vacancies in TbBaCo₂O₄.₅ influences its magnetic and electronic properties.
• Cationic and anionic lattice coupling offers a strategy for tuning the properties of LnBaCo₂O₅₊δ materials.