Ferroelectric Phase Transition in Ammonium Hydrogen Bis (Chloroacetate). IV. Heat Capacity of NH₄H(ClCH₂COO)₂ and ND₄D(ClCH₂COO)₂

Abstract

Heat capacity of NH₄H(ClCH₂COO)₂ and ND₄D(ClCH₂COO)₂ was measured between 3 and 300 K with an adiabatic calorimeter. It showed a lambda-shaped anomaly at 119.7 and 130.3 K for the H and D salts, respectively, with the same entropy of transition of about (Remark: Graphics omitted.). The isotope effects in chloroacetate anion were extracted by comparing the difference plot of normal heat capacity between two salts with that of NH₄Br and ND₄Br. A collective mode involving the short hydrogen bond in chloroacetate anion is indicated. The unusual behavior below 20 K suggested that the D crystal is harder than the H crystal, and also led to the characteristic properties in these crystals. Two possible mechanisms of phase transition, continuous displacive type and order-disorder type, are examined by the observed heat capacity. Simple and modified order-disorder models are ruled out.