Thermodynamic study on thermochromic phase transition in isopropylammonium trichlorocuprate

Abstract

Heat capacities of isopropylammonium trichlorocuprate(II) were measured in the range T=18–362 K with an adiabatic calorimeter. A thermochromic phase transition due to coordination geometry changes in CuCl 3− anion was observed above 325 K. This phase transition was easily superheated. In case of a continuous heating method in the adiabatic calorimeter, the transition from the low-temperature brown phase to the high-temperature orange phase occurred at 335.6 K, while the usual intermittent adiabatic method brought about the transition at 326.82 K. The transition enthalpy and entropy were 5.54 kJ mol −1 and 16.5 J K −1 mol −1, respectively, when the transition occurred at 335.6 K. The high-temperature phase was also easily undercooled. Easy superheating and undercooling phenomena, characteristic of the present phase transition, should be interpreted not by the energy difference but by the kinetics governing the transfer between two states. Heat capacities of the undercooled metastable state were also measured in the 14–258 K range. The specimen rapidly cooled from the high-temperature phase exhibited a higher-order phase transition without color change at 207.8 K, with the enthalpy and entropy change of 1.33 kJ mol −1 and 6.90 J K −1 mol −1, respectively. Spontaneous stabilization of the metastable state to the stable one occurred just above 258 K. Below the higher-order phase transition a glass transition phenomenon occurred at 154 K. A plausible quenched molecular mode below the glass transition may be the orientational mode of isopropylammonium cation.