N14NMR study of the incommensurate phase transition in(NH4)2BeF4

The temperature dependences of the quadrupole splitting and spin-lattice relaxation of the ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ NMR lines have been studied in the paraelectric, the incommensurate ($I$), and the ferroelectric phases of ${\left(\mathrm{N}{\mathrm{H}}_4\right)}_2$Be${\mathrm{F}}_4$. The characteristic broadening of the ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ lines in the incommensurate and the splitting in the ferroelectric phase demonstrate that the N(2)${\mathrm{H}}_4$ groups are the main reorientable ferroelectric dipoles in the structure though the N(1)${\mathrm{H}}_4$ and Be${\mathrm{F}}_4$ groups take part as well in the phase transition mechanism. The results further show that the incommensurate modulation has a soliton character close to $T_c$ and that the ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ spin-lattice relaxation in the $I$ phase is determined by phasons with a finite-energy gap.