Accurate Heat Capacity Measurement at First-Order Phase Transition by Relaxation Calorimetry System

Abstract

Relaxation calorimetry enables us to measure the heat capacity of small samples with high accuracy, particularly at low temperatures. However, extracting accurate heat capacities at first-order or very sharp second-order phase transitions is extremely difficult. As a key to overcome this challenge, we focused on the“scanning method”and introduced several corrections in the data analysis. Using this method, we made heat capacity measurements around first-order transition of KH₂PO₄ around 122 K and BaTiO₃ around 203 K by a commercial relaxation calorimeter PPMS (Physical Property Measurement System). The results for KH₂PO₄ were shown to be excellent; a very sharp peak in heat capacity was obtained and the absolute values were shown to agree well with the previous results obtained by adiabatic calorimetry on much larger samples. The critical behavior of the heat capacity in the vicinity of the transition temperature, as well as the thermodynamic quantities such as the transition enthalpy and entropy, also agreed very well with the previous results. For BaTiO₃, clear hysteretic behavior of the transition was observed for heating and cooling curves.