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

Topotactic reduction of the perovskite oxide TbBaCo2O5.5 with CaH2 leads to a new crystalline phase TbBaCo2O4.5, adopting a 2 × 2 × 1 superstructure compared to TbBaCo2O5.5. The structure consists of a corner-shared network of square pyramidal CoO5 and trigonal planar CoO3 units. Magnetic susceptibility and variable temperature neutron diffraction data reveal that TbBaCo2O4.5 adopts a G-type antiferromagnetically ordered structure (TN ∼ 322 K). The ordered moments are consistent with the presence of low-spin Co2+ (S = 1/2) in trigonal-planar coordination and high-spin Co2+ centers in square pyramidal coordination. TbBaCo2O4.5 shows lower conductivity than TbBaCo2O5.5, which is consistent with the p-type conduction behavior. The unique anion vacancy arrangements in TbBaCo2O4.5 further complement the role of A-cations in controlling the oxygen vacancy distribution in LnBaCo2O5+δ series and demonstrate more opportunity to tune the structural and physical properties based on cationic and anionic lattice coupling.