Heat capacity and ehrenfest relation at the ferroelectric phase transitions of rochelle salt

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Abstract
The heat capacity of a short-circuited Rochelle salt single crystal was measured with a precise conventional calorimeter with double adiabatic shielding. Below the upper Curie point we found a small positive, below the lower Curie point a small negative anomaly in the heat capacity. At each of the two transition temperatures the specific heat increased by only about 0.9% on going to the ferroelectric phase. The positive signs of the two steps are in accordance with general thermodynamics and existing theories of Rochelle salt. However, as a previous evaluation of the Ehrenfest relation had given contrary predictions, we discussed this relation based on more recent measurements; thus we could explain the contradiction in question and ascertain that the Ehrenfest relation seems to be valid at the two ferroelectric phase transitions of Rochelle salt.