Structural and magnetic characterization of the frustrated triangular-lattice antiferromagnets CsFe(SO4)(2) and RbFe(SO4)(2)

We describe the characterization, by magnetometry, neutron diffraction, and x-ray diffraction, of polycrystalline samples of CsFe(SO4)(2) and RbFe(SO4)(2). The crystal structures of these materials are of the trigonal CsSb(PO4)(2) (P (3) over bar) type, in which the magnetic Fe3+ ions with spin S=5/2 occupy a triangular lattice in well-separated layers. Comparison of the low-temperature magnetic susceptibility with Monte Carlo simulation results shows that both compounds approximate very well to the two-dimensional Heisenberg model antiferromagnet on the triangular lattice. At the Neel temperatures, 4.4 K for CsFe(SO4)(2) and 4.2 K for RbFe(SO4)(2), the spins order three dimensionally, although this does not have a strong effect on the form of the susceptibility curve. The ordered magnetic structures have been determined for both materials, revealing that the spins form a three-sublattice structure in the basal plane, with adjacent spins rotated by 120 degrees and with spins in alternate layers approximately antialigned. [S0163-1829(99)10121-8].