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(PO 4)(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 l ow-temperature magnetic susceptibility with Monte Carlo simulation results shows that both compounds approximate very well to the two-dimensional Heis enberg model antiferromagnet on the triangular lattice. At the Neel tempera tures, 4.4 K for CsFe(SO4)(2) and 4.2 K for RbFe(SO4)(2), the spins order t hree dimensionally, although this does not have a strong effect on the form of the susceptibility curve. The ordered magnetic structures have been det ermined for both materials, revealing that the spins form a three-sublattic e structure in the basal plane, with adjacent spins rotated by 120 degrees and with spins in alternate layers approximately antialigned. [S0163-1829(9 9)10121-8].