High-pressure Synthesis of Ba₂CoO₂Ag₂Te₂ with Extended CoO₂ Planes

Using a high-pressure synthesis method, we prepared the layered oxychalcogenide Ba₂CoO₂Ag₂Te₂ (space group: I4/mmm) with alternating stacks of CoO₂ and Ag₂Te₂ layers, separated by Ba atoms. The CoO₂ plane is greatly extended (Co-O = 2.19 Å on average) due to tensile strain from adjacent Ag₂Te₂ layers, causing displacement of oxide anions. Layered cobaltates with trans-CoO₄X₂ (X = chalcogen, halogen) coordination feature large spin-orbit coupling, which is linearly scaled by the tetrahedral factor of d_Co-X/d_Co-O. However, applying this relation to Ba₂CoO₂Ag₂Te₂ yields a magnetic moment of ∼4 μ_B, which is nearly twice the experimentally observed value of 1.87(17) μ_B. This result, along with a reduced Néel temperature (T_N = 60 K), originates from the off-centered position of otherwise under-bonded oxide anions, which changes the crystal field splitting of Co d orbitals.