Ferroelectric Phase Transition in Ammonium Hydrogen Bis (Chloroacetate) I. Nuclear Quadrupole Resonance in NH4H(ClCH2COO)2 and ND4D(ClCH2COO)2

Temperature dependence of the 35 Cl nuclear quadrupole resonance in NH 4 H(ClCH 2 CO 2 ) 2 and ND 4 D(ClCH 2 CO 2 ) 2 showed that there was a single line above 120 K and 130 K, respectively. Below these transition temperatures the spectrum consists of a pair of lines whose separation gradually increases at lower temperatures, being 2.17 times as large in the D salt as in the H salt. The resonance frequencies at 20.1 K were 35.1230 and 35.3680 MHz in the H salt and 35.034 and 35.566 MHz in the D salt. Infrared absorption spectra also showed splitting in the C–Cl stretching band below the transition temperature. Principal values of the electric field gradient tensors at chlorine sites were determined for the two phases of the H salt; the direction of the principal Z axes being to change asymmetrically as the transition point is traversed downwards. Two possible mechanisms, the continuous displacive transition and the order-disorder transition, were examined and shown to account equally well for the experimental results.