Study of antiferroelectric ammonium dihydrogen phosphate (ADP) by pulsed NMR

Proton NMR relaxation rates have been measured in ammonium dihydrogen phosphate (ADP) over a temperature range from 315 to 74°K. The longitudinal lab-frame relaxation rate $\frac{1}{T_1}$ is completely dominated by the thermally activated hindered rotations of the ammonium ions. At the antiferroelectric phase transition, $T_c\simeq 147{}_{}{}^{\circ }{}_{}{}^{}\mathrm{K}$, $\frac{1}{T_1}$ exhibits a 50% discontinuity and, for $T<\mathrm{}{T}_c$, develops a temperature-dependent anisotropy. The rotating-frame relaxation rates, $\frac{1}{T_{1\rho }}$, show not only an expected smoothly temperature-dependent baseline contribution associated with the same ammonium-ion reorientations which dominate $\frac{1}{T_1}$, but also some additional sharply temperature-dependent structures. Weak structures in $\frac{1}{T_{1\rho }}$ near room temperature appear due to impurity-diffusion effects and can be eliminated by annealing the ADP samples. More interesting is an extremely strong ($\sim 5\times {10}^2 invsec$) intrinsic phase-transition anomaly contributing to $\frac{1}{T_{1\rho }}$ for temperature very close to $T_c$. Analysis of the quantitative results indicates that (i) the ammonium ions reorient by two-fold rotations both above and below $T_c$. (ii) There is NMR evidence for severe ammonium-ion distortions and rotational anisotropies in the antiferroelectric state, but the phase transition itself does not involve cooperative rotational ordering of the ammonium ions. (iii) Critical slowing of some dynamical mechanism is strong reflected in the $\frac{1}{T_{1\rho }}$ anomaly near the phase transition; the associated correlation times become very long, $\sim {10}^{-6\mathrm{}} sec$, for temperatures just above $T_c$ and rapidly freeze-in to an essentially static behavior for temperatures below $T_c$.