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Thermodynamic studies of thermochromic phase transition in coordination compound of iron with triazole

The aim of this work was to investigate the thermodynamic properties of nitrate tris(4-amino- 1,2,4-triazole) iron(II) for which a thermochromic effect takes place at T equals 342 K on heating and at 310 K on cooling. For the measurement of the isobaric heat capacity in the 5 - 355 K range, a vacuum adiabatic calorimeter was used. Below 300 K, the Cp,m(T) curve has no anomalies; the average deviation of the experimental heat-capacity values from the smoothed Cp,m(T) curve was 0.1 percent. Above 300 K, the Cp,m(T) curve shows a complicated behavior. The abnormal part of the heat capacity first increases smoothly, then from 330 K a sharp rise begins. There is a peak on the Cp,m-curve at T equals 343.0 K. This fact is in a good agreement with the magnetic measurement data. The transition enthalpy was determined to be 22.8 kJ(DOT)mol-1 (M equals 432.10 g(DOT)mol-1). Further increase of the heat capacity suggests the existence of a new phase transition above 350 K. This suggestion was confirmed qualitatively by the measurements on a differential scanning calorimeter. It should be noted that on the low- temperature slope of the anomaly, at 320 < T < 330 K, there is a small anomaly with the maximum deviation of 5 - 10% from the regular part. To compare the phonon characteristics, the heat capacity of nitrate tris(4-amino-1,2,4-triazole) copper(II) was studied.