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Using a high-pressure synthesis method, we prepared the layered oxychalcogenide Ba 2 CoO 2 Ag 2 Te 2 (space group: I 4/ mmm ) with alternating stacks of CoO 2 and Ag 2 Te 2 layers, separated by Ba atoms. The CoO 2 plane is greatly extended (Co-O = 2.19 Å on average) due to tensile strain from adjacent Ag 2 Te 2 layers, causing displacement of oxide anions. Layered cobaltates with trans -CoO 4 X 2 (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 Ba 2 CoO 2 Ag 2 Te 2 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.

A diamond spin chain system, one of the one-dimensional frustrated lattices, is known to exhibit novel properties, but experimental studies have been exclusively confined to materials with a single spin component. Here, we report on the synthesis, structure, and magnetic properties of a new diamond chain compound Cu 2 FePO 4 F 4 (H 2 O) 4 1 composed of mixed-spins of Cu 2+ (S = 1/2 × 2) and Fe 3+ (S = 5/2). Compound 1 crystallizes in the space group C2/c of the monoclinic crystal system with a = 7.7546(4) Å, b = 12.1290(6) Å, c = 9.9209(6) Å, β = 105.29(1)°, and Z = 4. DC magnetization, Mössbauer spectroscopy, and heat capacity measurements revealed an antiferromagnetic order at 11.3 K with a small ferromagnetic component. It is suggested that ferrimagnetic diamond chains are arranged in an antiferromagnetic fashion (i.e., [...Fe(↑)-2Cu(↓↓)-Fe(↑)...] and [...Fe(↓)-2Cu(↑↑)-Fe(↓)...]) within the ab plane to cancel net magnetization, and the spin orientation of the diamond chains changes alternately along the c axis due to the magnetic anisotropy, leading to a noncollinear spin order. Furthermore, another anomaly is observed in the heat capacity at around 3 K, suggesting a successive magnetic transition or crossover due to competing magnetic interactions.