NMR Study on Ferroelectric (CH₃)₂NH₂H₂AsO₄

Abstract

The dimethylammonium dihydrogen arsenate (CH3)2NH2H2AsO4 (DMADA), which belongs to KDP-type crystals, undergoes a ferroelectric phase transition of second order at 274.5K (1⁄4 Tc). The phase transition is characterized by the symmetry change from the monoclinic-space group P21=n to Pn. 1) It was also reported that this crystal shows the drastic isotope effect on the phase transition temperature by deuteration. Moreover, the critical slowing-down phenomenon of polarization has been observed near Tc. 2) These facts suggest that the phase transition at 274.5K is of the order– disorder type and that the hydrogen plays an important role in the phase transition of DMADA. In addition to these properties, this crystal shows the unique and interesting features that spontaneous polarization increases slowly with decreasing temperature below Tc and is not saturated even at 77K. On the other hand, the spontaneous polarization is saturated at a temperature T 1⁄4 Tc 10K in the KH2AsO4 crystal. The replacement of the K ion of KDP family with the dimethylammonium ion will lead to such a tempeature dependence of spontaneous polarization in DMADA. Therefore, investigations on not only hydrogen dynamics at the order–disorder phase transition of 274.5K but also the motion of dimethylammonium ion will be helpful for generalizing the ferroelectricity and ferroelectric phase transition of KDP-type crystals. Recently, we have carried out H-NMR measurements for DMADA. In the present paper, we report the dynamics of hydrogen from the results of the spin–lattice relaxation and the NMR line measurements in DMADA. The DMADA single crystals were grown by slow evaporation method at 303K from an aqueous solution containing (CH3)2NH2 and H3AsO4 in equimolar ratio. The measurement of the H spin–lattice relaxation time T1 was performed using pulse spectrometer (Thamway Prot1002MR) with the frequency of 10.5MHz. The spin–lattice relaxation time was obtained from the recovery of the intensity of the spin echo signal following the saturation comb pulses. The H-NMR lines were observed with the NMR spectrometer by the Q meter detection method. In the NMR measurement, the powder specimens were used. The temperature dependence of H spin–lattice relaxation ratio at 10.5MHz is shown in Fig. 1. The spin–lattice relaxation rate T 1 1 takes a maximum at about 107K, and a sharp peak at 275K (see the inset of Fig. 1). The anomaly of T 1 1 near 275K is characterized by a very sharp peak. This crystal undergoes the phase transition of the order–disorder type at 274.5 K. Therefore, this anomaly of T 1 1 accompanies the phase transition observed at 274.5K. Moreover, we note that T 1 1 shows the change from 83.3 s to 0.285 s below room temperature. In the case that the rotation of methyl group exists, T 1 1 is given by