The heat capacity and the magnetic susceptibility between 1 and 33 K of antiferromagnetic ${\mathrm{Rb}}_2$${\mathrm{FeCl}}_5$⋅${\mathrm{H}}_2$O with spin S=(5/2) are reported. Examination of the possible superexchange paths and analysis of the data allow us to conclude that the magnetic lattice presents low dimensionality at higher temperatures and then orders three dimensionally at 10.0±0.02 K. The experimental results have been fitted to a Heisenberg model with two effective intrachain and interchain exchange interactions. Theoretical values for the thermodynamic functions and characteristic fitting parameters $J_z^{\mathcal{’}}$/$k_B$$T_c$, ${J_z^{\mathcal{’}}/k_B\mathit{T}(\mathit{\chi }}_{\max }$), and ${\mathit{\chi }}_{\max }$ have been calculated by means of high-temperature-series analysis. The best fit gives $J_z^{\mathcal{’}}$/$k_B$=-1.45±0.06 K and $J_{\mathrm{xy}}^{\mathcal{’}}$/$k_B$=-0.15±0.07 K. Spin-wave theory has been extended to this particular case of magnetic lattice crossover, the parameters resulting being in agreement with those from the paramagnetic region. A comparative analysis of the isostructural series $A_2$${\mathrm{FeCl}}_5$⋅${\mathrm{H}}_2$O (A=K, ${\mathrm{NH}}_4$, Rb) is carried out within the same theoretical framework.

• Received 25 May 1984

DOI:https://doi.org/10.1103/PhysRevB.31.516