NMR studies of phase transition in ammonium hydrogen bischloroacetate $NH_4H(CH_2ClCOOO)_2$

Proton second moment $(M_2)$ and spin-lattice relaxation time $(T_1)$ of Ammonium Hydrogen Bischloroacetate (ABCA) have been measured in the range 77-350 K. A value of $6.5 G^2$ has been observed for the second moment at room temperature, which is typical of $NH_4^+$ reorientation and also a second moment transition in the range 170-145 K indicates the freezing of $NH_4^+$ motion. The NMR signal disappears dicontinuously at 128 K. Proton spin-lattice relaxation time $(T_1)$ Vs temperature, yielded only one sharp miniumum of 1.9 msec which is again typical of $NH_4^+$ reorientation. A slope change at 250 K is also observed, prbably due to $CH_2$ motion. Further, the FID signal disappears at 128 K. Thus the $T_c$ appears to be 128 K (of two reported values 120 K and 128 K). Activation energies have been calculated and the mechanism of the phase transition is discussed