27Al and71Ga NMR Measurements on Single Crystals of DMAAS and DMAGaS

Abstract

Single crystals of dimethylammonium aluminium sulfate (DMAAS) and dimethylammollium gallium sulfate (DMAGaS) were investigated in the high temperature (normal) phase and near the ferroelectric phase transitions by means of quadrupolar perturbed 27 Al and 71 Ga NMR spectroscopy, respectively. The electric field gradient (EFG) tensors were determined at ambient temperature from the quadrupole splittings of first order. The first-order quadrupole line splittings in the 27 Al and 71 Ga NMR spectra for the DMAAS and DMAGaS crystals, respectively, measured in the high temperature paraelectric phases. only differ by a scaling factor of 1.66. They confirm that both crystals are isomorphous in the structure. Near the phase transition it about 136 K step-like changes occur in the 71 Ga NMR spectra of DMAGaS crystals which are consistent with the first-order type of the phase transition. For DMAAS a second-order transition at about 153 K to the ferroelectric phase can be clearly inferred from the 27 Al NMR spectra. The frequency change below this second-order phase transition can be described by a power law with an exponent close to 0.5. These conclusions about the phase transitions are quite consistent with the results of ultrasonic measurements and of Raman spectroscopic studies. Inspite of the sufficiently accurate experimental results, however, a more detailed interpretation of these results is not yet possible because the nature of the coupling between the NMR frequency changes below T c and the order parameter is not yet clear.