Ferroelectric Phase Transition in Ammonium Hydrogen Bis (Chloroacetate). III. Chlorine-35 Quadrupole Relaxation

Abstract

The ³⁵Cl nuclear quadrupole resonance frequencies and relaxation times in ammonium hydrogen bis (chloroacetate) were measured in the ferro- and the paraelectric phases. The splitting of the resonance line in the ferroelectric phase shows the temperature dependence in the form of Δν=(T_c−T)^1⁄3 near the transition point T_c. Sharp decrease of T₁ has been observed in the critical region and it was attributed to the critical slowing down of the electric dipolar interactions. Two kinds of relaxation mechanisms were examined: The order-disorder type of mechanism can explain the T₁ data of the paraelectric phase but it fails to explain the different temperature dependence of T₁ of ν₁ and ν₂ in the ferroelectric phase. The other mechanism, the displacive-mode transition is plausible because it gives a reasonable description of the T₁ anomaly both in the ferro- and paraelectric phases. This mechanism is also consistent with the structural evidences.