Abstract
Deuterated single crystals of betaine phosphate (BP) and betaine phosphite (BPI) are investigated by means of 2H 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 ∝ |Tc – T|β a value β 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.
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