Ordering behaviour at the antiferrodistortive phase transition in betaine phosphate and betaine phosphite

Deuterated single crystals of betaine phosphate (BP) and betaine phosphite (BPI) are investigated by means of 2 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 ∝ |T c - 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.