C NMR of DIPAC and DIPAB organic ferroelectrics.

The diisopropylammonium chloride (CHClN, DIPAC) and diisopropylammonium bromide (CHBrN, DIPAB) molecular crystals are recently discovered ferroelectrics with sufficiently high spontaneous polarization and Curie temperature. We performed first studies of these crystals by C NMR. CP MAS spectra were collected within large temperature ranges covering the Curie points. The reconstructive phase transition from the initial orthorhombic P222 structure of DIPAB to the monoclinic ferroelectric P2 structure leads to an abrupt alteration in the C spectrum. The C spectra for DIPAC and DIPAB in the ferroelectric P2 phase are quite similar with four lines at lower frequencies, which correspond to the CH groups, and two lines with close chemical shifts, which correspond to two CH groups. The transition into the paraphase leads to gradual reduction of the interline distances in the low-frequency quadruplet and in the doublet. The step-like changes in the interline frequency shifts at this transition indicating its first order. The analysis of the spectrum evolution in the paraphase shows that only a CH group lays in the reflection plane above the P2  →  P2/m transition, while the second CH group only moves closer to the reflection plane upon further heating.