Chemical exchange processes of deuterons in partially deuterated betaine phosphate (DBP) and betaine phosphite (DBPI) crystals are investigated by means of NMR experiments over a wide temperature range. The lineshape analysis of one-dimensional (1D) NMR experiments on DBP reveals a behaviour which is characteristic for exchange processes between all deuterons in the hydrogen bonds. In the temperature range between 230 and 310 K two-dimensional (2D) NMR exchange spectroscopy has been applied, revealing a uniform rate of exchange between all sites. The exchange rates for both crystals are found to show an Arrhenius-like behaviour with activation energies of 83 and , respectively, where for temperatures above 360 K the rates are obtained from the lineshape analysis of 1D NMR spectra. In addition, conductivity measurements for DBP and DBPI are reported. The dc conductivity in the direction of the quasi-one-dimensional chains, formed essentially by the phosphate and phosphite groups in DBP and DBPI, is characterized by activation energies of 115 and , respectively. For DBPI this value is close to that obtained for the exchange processes by means of NMR, whereas a difference of about is found in the case of DBP. Quantitative relations between the exchange and conductivity processes are established and, on the basis of simplifying assumptions, an estimation of effective charge-carrier densities is discussed.
Deuterated single crystals of betaine phosphate (BP) and betaine phosphite (BPI) are investigated by means of 'H NMR measurements in order to study the behaviour at the antiferrodistortive phase transitions. The experimental data are interpreted in terms of a microscopic order-disorder model. For the temperature dependence of the order parameter according to p 0: IT,-TIp a value p was found to be close to 0.25 for both crystals. In accordance with measurements of the heat capacity, this points to a behaviour which is similar to the case of a tricritical point. Our data can also be explained on the basis of the assumption of a temperature dependent fourth-order coefficient in the Landau free energy as has been recently discussed theoretically.
Studies of the electrical conductivity in solid solutions of the mixed crystal system of deuterated antiferroelectric betaine phosphate (DBP) and ferroelectric betaine phosphite (DBPI) were performed. The dc-conductivity in the temperature interval between 280 and 400 K is obtained from measurements of the complex dielectric permittivity at frequencies between . For all crystals studied an anisotropy of with respect to two crystallographic directions is found. Moreover, we find a clear dependence of the absolute values of on the phosphite percentage x, whereas the activation energies calculated from an Arrhenius plot of the quantity against the inverse temperature differ only slightly. For the system the conductivity results are related to data on the microscopic behaviour of the deuteron exchange derived from two dimensional nuclear magnetic resonance (NMR) exchange spectroscopic measurements. An effective charge carrier density is estimated and discussed with respect to the differences between the conductivity and the exchange behaviour for pure deuterated substances.
Processes of chemical exchange of deuterons in partially deuterated ammonium hydrogen selenate, NH4HSeO4 (AHSe), crystals are investigated by means of 2H nuclear magnetic resonance (NMR) experiments over a wide temperature range. The temperature dependencies of the quadrupole line splittings in the one-dimensional spectra of AHSe above 350 K revealed line-shape changes which are characteristic for chemical exchange processes. A detailed study of these exchange processes in AHSe is achieved by means of two-dimensional 2H NMR experiments. In the temperature range investigated, a chemical exchange occurs only between those deuteron (proton) sites which are involved in hydrogen bonds (- and -positions). It was established that the rates of exchange between all types of hydrogen-bound deuteron are approximately the same. Exchange between these positions and the deuterons in the ND4 groups could not be detected. On the basis of our findings, we finally discuss a model for the microscopic mechanism of hydrogen transport in AHSe.
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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