Structural Disorder, Octahedral Coordination, and 2-Dimensional Ferromagnetism in Anhydrous Alums

TL;DR

This study characterizes the crystal structures and magnetic properties of layered anhydrous alums KCr(SO4)2, RbCr(SO4)2, and KAl(SO4)2, revealing 2D ferromagnetism and structural disorder in their layered frameworks.

Contribution

It provides detailed structural and magnetic analysis of layered anhydrous alums, highlighting the role of disorder and octahedral coordination in 2D ferromagnetism, which was not previously characterized in these compounds.

Findings

All compounds crystallize in space group P-3 with octahedral coordination.

Structural disorder involves stacking faults and layer rotations.

Magnetic order is ferromagnetic within layers and antiferromagnetic between layers below 3 K.

Abstract

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 at temperatures between 1.4 and 773 K. The compounds all crystallize in the space group P-3, 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 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.