Revised Bi/M Layered Oxo-Sulfate (M = Co, Cu): A Structural and Magnetic Study

The combination of oxo-bismuthite slabs with counter oxo-anions main often leads to a series of layered Sillen-related compounds. For electroneutrality, it is convenient that the structure incorporates aliovalent cations such as Bi³⁺/M²⁺ ions that could be confined either in the Bi/O slabs or in the interleave. In the case of SO₄^2– oxo-anions, only two compounds have been reported for M = Co, Cu, while interesting magnetic topologies emerge in absence of any prior characterization. Our reinvestigation of these systems using crystal growth in Bi₂O₃ flux led to the full structural characterization of [Bi₂CoO₃]­(SO₄) (I) and [Bi_6.2367Cu_1.6O8]­(SO₄)₃ (II), which both exhibit pleated [Bi/M/O] slabs running in doubled cells comparatively to previous reports. (I) shows an interesting weak ferromagnetism (∼0.2 μ_B/Co) below 17 K arising from Dzyaloshinskii–Moriya interactions in Co²⁺ zigzag chains, while (II) is essentially a paramagnet despite the presence of Cu₂O₆ dimers. Finally in the novel [Bi₆O₆]­(CoO₂)­(SO₄)₂ (III), the Co²⁺ ions are ordered with sulfate in the interleaves, leading to a noncentrosymmetric crystal structure