Magnetic specific heat of the low-temperature phase of rubidium manganese hexacyanoferrate

In the specific heat measurement of RbIMnIII[FeII(CN)6] (MnIII; S=2, FeII; S=0), an anomalous peak due to a long-range magnetic ordering was observed at 11.0 K (Tp). The magnetic transition entropy (ΔSmag) and enthalpy (ΔHmag) were evaluated from the magnetic specific heat (Cmag) to be 11.8 ± 0.9 J K−1 mol−1 and 125 ± 9 J mol−1, respectively. A detailed analysis of Cmag, ΔSmag, ΔHmag, and Tp has shown that the present magnetic phase is a three-dimensional Heisenberg-type ferromagnetic lattice of MnIII sites with an exchange coupling constant of ≈+0.5 cm−1. The ferromagnetic ordering is ascribed to the valence delocalization mechanism.