X-RAY-INVESTIGATIONS OF THE HIGH-TEMPERATURE PHASE-TRANSITION OF BETAINE PHOSPHATE

The critical behaviour of the high temperature phase transition (T(c) = 365 K) of betaine phosphate was investigated by two different X-ray methods: the double crystal technique and the energy dispersive Laue method. We measured the temperature dependence of the order parameter, the thermal expansion, and the intensity of the critical scattering. The phase transition can be characterized as a second order transition which is very close to a tricritical one. The critical exponents correspond to those of the 3-dimensional Ising model within the accuracy of our measurements which is limited by the sample quality.