A ¹⁴N nuclear quadrupole resonance study of phase transitions and molecular dynamics in hydrogen bonded organic antiferroelectrics 55DMBP-H₂ca and 1,5-NPD-H₂ca

The temperature dependence of the ¹⁴N NQR frequencies has been measured in antiferroelectric and paraelectric 55DMBP-H₂ca and 1,5-NPD-H₂ca. In both compounds we observe two non-equivalent nitrogen positions (N⁺-H⋯O^- and N⋯H-O) in the antiferroelectric phase. The two nitrogen positions become equivalent (N⋯H⋯O) in the paraelectric phase. The critical exponent of the local antiferroelectric order parameter has been determined from the NQR data. The principal values of the quadrupole coupling tensor correlate in both compounds. The correlation diagrams clearly show how a proton migrates from the antiferroelectric position towards the paraelectric position in the bifurcated hydrogen bond on increasing the temperature. A slow motion has been observed in 55DMBP-H₂ca by the ¹H and ¹⁴N spin-lattice relaxation. An analysis of the spin-lattice relaxation data suggests a slow exchange between two non-planar conformations of the bipyridine molecule.