NMR detection of dynamical processes in antiferroelectric nanoclusters during the order-disorder transition in ${\mathrm{NH}}_4{\mathrm{H}}_2{\mathrm{AsO}}_4$

We study the dynamics of inorganic antiferroelectric nanoclusters formed during an order-disorder transition and demonstrate the coexistence of the two phases in a region of 2–3 K around the transition temperature $T_N\sim 215$ K. Single crystals of ${\mathrm{NH}}_4{\mathrm{H}}_2{\mathrm{AsO}}_4$, a model hydrogen-bonded compound, show an antiferroelectric-paraelectric transition studied by means of highly sensitive magic angle spinning ${}^{15}\mathrm{N}$ NMR at 21.1 T. The phase coexistence is demonstrated by a double-peak structure of the chemical shift. Two-dimensional chemical exchange spectroscopy and spin-lattice relaxation time $\left(T_1\right)$ measurements show that the clusters are dynamic with sizes $\sim 50$ nm and lifetimes approaching seconds as $T\to T_N$. Their occupancy increases rapidly to fill the crystal volume below $T_N$. This study provides evidence for the commonality of the phase transitions in systems with electric properties and provides an improved spectroscopic method for such studies.

Figure 1

(a) ${}^{15}\mathrm{N}$ isotropic chemical shift of ADA, showing an abrupt change at $T_N=215.2$ K, as a function of temperature. Data points indicate the maxima of ${}^{15}\mathrm{N}$ CPMAS spectra, such as shown in (b). (b) Contour plot of CPMAS spectra (arbitrary units) in the vicinity of $T_N$, with darker areas indicating the location of maxima or high signals. The insets show three spectra; the vertical axes are of same range for all three and the horizontal axis is identical to the one of the contour plot. $T_N$ was defined as the temperature at which the PE (dark gray) and AFE (light red) peaks have equal integrated intensities.

Figure 2

(a) Normalized ${}^{15}\mathrm{N}$ magnetization $M_z^{\text{CP}}$ of the PE (squares) and AFE (dots) phases vs the inversion recovery time $\tau$ at $T_N$. Single exponential fittings (lines) yield $T_1$ of 1.03 and 0.82 s for the PE (black) and AFE (red) phases, respectively. The inset is a zoom of data in (b) as indicated by the blue rectangle. (b) Temperature dependence of ${}^{15}\mathrm{N}{T}_1$ in ADA shown in logarithmic scale for the vertical axis and reciprocal scale $\left(1/T\right)$ for the temperature axis, to put in evidence the linear behavior [see Eqs. (2) and (3)].

Figure 3

Correlation time as a function of temperature, for the AFE (dots) and PE (squares) phases shown in logarithmic scale for the vertical axis and reciprocal scale $\left(1/T\right)$ for the temperature axis. The lines show Arrhenius fits done using Eq. (3). The inset shows the same data points (no fits) in the same representation, inside the order-disorder transition (error bars are smaller than dot sizes).

Figure 4

(a) 2D ${}^{15}\mathrm{N}$ exchange spectrum of ADA at 215.2 K with a mixing time of 1 s. The presence of the cross peaks indicates a dynamic exchange of the nuclei from the AFE clusters to the PE clusters. (b) Schematic of the AFE clusters in the PE clustered lattice in going from the PE phase (black) to the AFE phase (red).