Synthesis, structures and magnetic properties of pseudo-hollandite chromium sulfides

The pseudo-hollandite chromium sulfides, ACr5S8 (A=K, Rb, Cs) and A′0.5Cr5S8 (A′=Sr, Ba), have been synthesized and investigated in structural and magnetic properties. All the compounds crystallize in the isostructure with a monoclinic C2/m. Its crystal structure has triangular lattices and double chains made of Cr3+ (d3; S=3/2) triangles. The magnetic susceptibilities of all compounds behave as Curie–Weiss types in high temperature region. From magnetic susceptibility and specific heat measurements, all the compounds have antiferromagnetic ground states. The Néel temperatures are rather low compared to Weiss temperatures, reflecting magnetic frustration in the triangular lattices and double chains. The magnetic transitions in KCr5S8 and Ba0.5Cr5S8 are a two-step transition around 50 and 60 K, respectively, while RbCr5S8 shows a sharp magnetic transition at 42 K, accompanied by magnetoelastic behavior. CsCr5S8 shows a structural transition around 100 K, followed by a magnetic transition at 10 K. In Sr0.5Cr5S8, ferromagnetic interaction develops below 100 K and then a three-dimensional antiferromagnetic order takes place at 30 K. These magnetic properties are discussed from A-cation dependences of local structures and a model of magnetic structure for RbCr5S8 is proposed on the basis of the arguments of magnetic interactions and neutron diffraction data. It is different from the known magnetic structure of TlCr5Se8.