The Ferroelectric Phase Transition in (Glycine)3·H2SO4 and Critical X-Ray Scattering

Abstract

On the assumption that the ferroelectric phase transition in (Glycine) 3 ·H 2 SO 4 is of the order-disorder type, expressions for Bragg reflections and critical scattering of X-rays are given in terms of a long-range order parameter and pair correlation functions. Temperature dependence of the critical scattering is discussed on the basis of the Bragg-Williams approximation and of a modified Frohlich theory. Experimental observations agree with theoretical predictions, and prove that the phase transition is of the order-disorder type. The observed critical scattering exhibits a pronounced peak at the Curie point, suggesting that the local field theory is not a good approximation. The shape of the peak appears to be quite different from that in the ferromagnetic case. It has been found that the sulfur atoms do not shift appreciably when the glycine groups rotate. A method for evaluating the pair correlation function is proposed.