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 SO42– 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 [Bi6.2367Cu1.6O8]­(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
μB/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.