Nuclear Magnetic Resonance and Nonexponential Spin-Lattice Relaxation in Ferroelectric Ammonium Fluoroberyllate

Nuclear spin-lattice relaxation times $T_1$ and relaxation times along the rf field ($T_{1\rho }$) have been measured for ${\mathrm{H}}^1$ and ${\mathrm{F}}^{19}$ in ${\left(\mathrm{N}{\mathrm{H}}_4\right)}_2$Be${\mathrm{F}}_4$. Below 250°K the relaxation is due to reorientation of N${\mathrm{H}}_4^{+}$ ions; proton $T_1$ versus $T$ has two minima due to two inequivalent N${\mathrm{H}}_4^{+}$ ions in the unit cell. Above the transition temperature $T_c$ (176°K) and below 130°K, the logarithm of the correlation time ${\tau }_c$ is a linear function of $T^{-1\mathrm{}}$ with normal values of pre-exponential factors. Immediately below $T_c$, the time ${\tau }_c$ is anomalously short, as found previously in ${\left(\mathrm{N}{\mathrm{H}}_4\right)}_2$S${\mathrm{O}}_4$. Above 250°K, both proton and ${\mathrm{F}}^{19}$ relaxation become nonexponential and may be characterized by the same pair of relaxation times ${T_1}^{\prime }$ and ${T_1}^{\prime \prime }$. The shorter component (${T_1}^{\prime }$) is ascribed to H-F dipolar interactions, and the longer component (${T_1}^{\prime \prime }$) to H-F, H-H, and Be-F dipolar interactions. On deuteration, only the longer component remains; also, ${T_1}^{\prime }$ and ${T_1}^{\prime \prime }$ depend only on the rate of reorientation of Be${\mathrm{F}}_4$ tetrahedra. Deuteron-resonance results indicate that the dipole moment per N${\mathrm{H}}_4^{+}$ ion in ${\left(\mathrm{N}{\mathrm{H}}_4\right)}_2$Be