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.

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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.

Single crystals of betainc phosphitc and bctainc phosphate were investigated in the antiferrodistortive and paraelastic phases. The comparison of the data obtained in these phases indicates an order-disorder type of phase transition. It could be shown that the t,wo betaine molecules, which can be distinguished by 'H NMR, are located in the high-temperature phase in the mirror plane only in a statistical serise arid tilt by about *7.8" in opposite direction out of this mirror plane below the phase transition.Betainphosphit-und Betairiphosphateinkristalle wurden rnit Hilfe der 'H-NMR in der antiferrodistortiveri und der paraelastischen Phase untersucht. Ein Vergleich der IlleBergebnisse in diesen Phasen lassen auf eiri Ordriurigs-Unordnungs-Verhalten am Phasenubcrgang schliefien. Die Betairiphosphat-und Betairiphosphitmolekiile besetzeri die scheinbar rntstandene Spiegelebene nur irn Zeitmittel. Die Verkippung der Betairirriolekule in Bezug auf die Spicgclcbcnc bctragt beim Uhergang in die antiferrodistortive Phase ungefahr *7.8" in entgegengcsctzter Richtung.') Linnkstr. 5 ,