Deuteron Nuclear-Magnetic-Resonance Study of the Ferroelectric Phase Transition in Deuterated Triglycine Sulfate and KD2PO4

A small, but abrupt, change in the deuteron nuclear-magnetic-resonance spectrum in K${\mathrm{D}}_2$P${\mathrm{O}}_4$ has been observed at the Curie temperature $T_c$. Above $T_c$, the deuterons jump back and forth along the bond with a time between jumps ${\tau }_J\ll {10}^{-5\mathrm{}}$ sec. Within $\simeq 1$°C below $T_c$, ordering of deuterons in the bond is complete, with ${\tau }_J>{10}^{-3\mathrm{}}$ sec. Coexistence of distinct high- and low-temperature spectra over a small temperature interval at $T_c$ is evidence for a first-order transition. In contrast, the deuteron spectrum of N$\mathrm{D}_3^{}{}_{}{}^{+}$ ions in deuterated triglycine sulfate undergoes a continuous change with temperature at $T_c$ with no evidence for coexistence of phases. This is characteristic of a second-order transition. Evidence is presented for a displacive, rather than order-disorder, transition for glycine molecules. Electric-field-gradient tensors are evaluated for the deuteron sites above and below $T_c$ in both crystals.