Magnetic ordering in the compounds RbFeCl₃·2H₂O and CsFeCl₃·2H₂O

Mössbauer experiments have been performed in the magnetically ordered phase of RbFeCl₃·2H₂O and CsFeCl₃·2H₂O. Magnetic ordering is observed in both compounds, which are reported to show low-dimensional interactions. The observed temperature dependence of the hyperfine field of RbFeCl₃·2H₂O fits well to the S′ = 1 2 rectangular Ising model, with exchange constants |J₁/k| = 50±7 K and |J₂/k| = 0.40±0.25 K, and T_N = 12.08±0.06 K. The low-temperature orientation of the hyperfine field of RbFeCl₃·2H₂O has been determined by measurements on a single-crystal absorber in an applied field. The hyperfine field is oriented parallel to the a axis, which deviates slightly from the earlier reported magnetic moment direction at 16° to the a axis. This deviation is attributed to the dipolar contribution in the hyperfine field. The low value of the saturation hyperfine field (-7.5T for RbFeCl₃·2H₂O) and the relative importance of the dipolar contribution are discussed in relation to the crystal field effects of a tetragonally distorted octahedron.