A surprisingly large thermal hysteresis loop in a reversible phase transition of RbxMn[Fe(CN)6](x+2)/3•zH2O

A series of RbxMn[Fe(CN)(6)]((x+2)/3).zH(2)O compounds (x = 0.94 (1), x = 0.85 (2), and x = 0.73 (3)) showed a temperature-induced phase transition with a thermal hysteresis loop, which is caused by the charge transfer from Mn-II to Fe-III and a Jahn-Teller distortion of the produced Mn-III. The transition temperature T-p and the width of a thermal hysteresis loop DeltaT systematically changed with decreasing Fe/Mn ratio of (x + 2)/3; i.e., the T-p value decreased (T-p = 259 K (1), 236 K (2), and 205 K (3)) and the AT value increased (AT = 86 K (1), 94 K (2), and 116 K (3)). It is noteworthy that a surprisingly large DeltaT value of 116 K was recorded for 3. The trends of the T-p shift and DeltaT expansion were understood by the variation in the transition enthalpy based on the thermodynamical analysis. The crystal structures of the low-temperature (LT) phase for 1 and 2 were tetragonal structures, while the LT phase for 3 had a cubic structure. This difference is because the Jahn-Teller elongation of Mn-III is randomly oriented due to a low Mn-III/(Mn-II + Mn-III) ratio in the LT phase for 3.