NMR and NQR Study of Above-Room-Temperature Molecular Ferroelectrics Diisopropylammonium Chloride and Diisopropylammonium Perchlorate

摘要

The phase transitions and molecular motions in organic ferroelectrics diisopropylammonium chloride and diisopropylammonium perchlorate have been studied by proton NMR and spin-lattice relaxation and by N-14 NQR and spin-lattice relaxation. In diisopropylammonium chloride, proton T-1 shows slowing of the reorientation of methyl groups and a discontinuous decrease at the ferroelectric to paraelectric phase transition. The N-14 NQR frequencies are observed in the range characteristic for a C-NH2+-C ion. Diisopropylammonium perchlorate undergoes three phase transitions at varying temperatures. Proton and N-14 T-1 shows the presence of two slow motions: the reorientation of the methyl groups and a second slow motion in phases II and III which is most probably tumbling of diisopropylammonium ions. The discontinuous change of proton T-1 and M-2 at the ferroelectric to paraelectric phase transition is ascribed to exchange between two orientations of a diisopropylamrnonium ion in the paraelectric phase. The N-14 quadrupole coupling constant is in diisopropylammonium perchlorate for a factor of similar to 2 lower than expected for a C-NH2+-C ion. This is most probably the effect of the tumbling motion of the ion. The N-14 quadrupole coupling constant changes only slightly through phases I-III, whereas at the II -> I phase transition the asymmetry parameter eta strongly decreases due to the reorientation of the diisopropylammonium ion. No reorientational tunneling of the methyl groups is observed at low temperature.