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One- and two-dimensional 2H NMR measurements on selected single crystals of deuterated betaine phosphate/phosphite mixed crystals (DBP1-xDBPIx) in the antiferrodistortive and paraelectric phases were performed in order to study the static and dynamic behaviour of the deuterium atoms in the hydrogen bridges and in the methylene groups. In the 2H nuclear magnetic resonance (NMR) spectra of the mixed crystals, it is possible to distinguish between DBP and DBPI molecules. Furthermore, for all deuterons the electric field gradient (EFG) tensors could be determined. A characteristic dependency of the high-temperature phase transition on the phosphite concentration x was derived from the changes of the one-dimensional 2H NMR spectra. Our suggested model, which describes this dependency qualitatively, underlines the antiferrodistortive character of this transition. Furthermore, from the line-shape variations, the occurrence of processes of exchange between several deuterons, including DBP and DBPI molecules, was concluded. The presence of exchange between different molecules in the chains was confirmed by means of analysing the cross-peaks of two-dimensional 2H NMR exchange experiments. The observation of this charge-transport process along the chains from the microscopic point of view is very important for the understanding of electrical conductivity processes in these crystals.

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