LOW-TEMPERATURE ANTIFERROMAGNETIC BEHAVIOR OF ALPHA-RB2FEF5.H2O AND ALPHA-CS2FEF5.H2O

D.C. susceptibility and zero field Mossbauer spectroscopy measurements are used to characterize the low-temperature magnetism of alpha-Rb2Fe F5 . H2O and alpha-Cs2FeF5 . H2O. The compounds are rigorously isotypi c quasi-1D Heisenberg magnets based on hydrogen bonded chains of -{FeF 5H2O}2--pseudo-octahedra with intervening Rb+ and Cs+ cations that enh ance the low dimensionality. The 1D antiferromagnetic effects appear c omparable, T(chimax)(1D) approximately 5 K, while their 3D ordering te mperatures are quite different, T(N)(Rb+) = 4.0 K, T(N)(Cs+) = 2.7 K. The internal hyperfine fields extrapolated to T = 0 K are approximatel y 51 T suggesting zero point spin reduction (approximately 15%) consis tent with 1D behavior. High field d.c. susceptibility measurements for polycrystalline Cs2FeF5 . H2O suggest spin-flop behavior with H(SF) < 1 T. A qualitative comparison of the low-temperature magnetic propert ies of the F- and Cl- analogues for the series M2FeX5 . H2O (M = NH4+, K+, Rb+, Cs+) is given. (C) 1994 Academic Press, Inc.