A Surprisingly Large Thermal Hysteresis Loop in a Reversible Phase Transition of Rb_xMn[Fe(CN)₆]_(x+2)/3·zH₂O

A series of Rb x Mn[Fe(CN) 6 ] ( x +2)/3 · z H 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 Δ T 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 Δ T value increased (Δ T = 86 K ( 1 ), 94 K ( 2 ), and 116 K ( 3 )). It is noteworthy that a surprisingly large Δ T value of 116 K was recorded for 3 . The trends of the T p shift and Δ T 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 .