Nuclear magnetic resonance of ferroelectric NaNH4C4H4O6⋅4H2O and NaND4C4H2D2O6⋅4D2O

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Abstract
Deuteron NMR spectra, and proton and deuteron spin–lattice relaxation times have been studied on both sides of the paraelectric–ferroelectric transition in ammonium Rochelle salt, NaNH4C4H4O6⋅4H2O. Complete rotation patterns obtained near 300 K for the deuteron resonance of D2O are successfully interpreted in terms of the crystal structure of ordinary Rochelle salt with the assumption that all four D2O molecules are undergoing 180° flip motion. The deuteron resonance spectra of ND+4 are attributed to rapidly reorienting and distorted ND+4 tetrahedra which undergo a radical change at the Curie temperature (122 K) indicating the onset of a first order phase transition. The spontaneous polarization computed from the distortion of ND+4 determined by NMR accounts for 52% of the total polarization of ARS. Proton and deuteron relaxation times are interpreted in terms of a relatively fast reorientation of the NH+4 tetrahedra about their twofold axes accompanied by a relatively slow reorientation about their threefold axes. Below about 130 K the threefold motion becomes ineffective in producing relaxation, but the deuteron resonance line splittings remain relatively small down to 77 K, the lowest temperature of measurement. No deuteron resonance spectra typical of ND+4 tetrahedra rotating about their twofold axes were observed, probably because of the presence of quantum mechanical tunneling.