Structural disorder, octahedral coordination and two-dimensional ferromagnetism in anhydrous alums

The crystal structures of the triangular lattice, layered anhydrous alums KCr(SO4)2, RbCr(SO4)2 and KAl(SO4)2 are characterized by X-ray and neutron powder diffraction (NPD) at temperatures between 1.4 and 773 K. The compounds all crystallize in the space group image, with octahedral coordination of the trivalent cations. In all cases, small amounts of disorder in the stacking of the triangular layers of corner sharing MO6 octahedra and SO4 tetrahedra is seen, with the MO6−SO4 network rotated in opposite directions between layers. The electron diffraction study of KCr(SO4)2 supports this model, which on an average can be taken to imply trigonal prismatic coordination for the M3+ ions; as was previously reported for the prototype anhydrous alum, KAl(SO4)2. The temperature-dependent magnetic susceptibilities for ACr(SO4)2 (A=K, Rb, Cs) indicate the presence of predominantly ferromagnetic interactions. Low-temperature powder neutron diffraction reveals that the magnetic ordering is ferromagnetic in-plane, with antiferromagnetic ordering between planes below 3 K.