Order-disorder behavior in betaine arsenate studied by Raman scattering

A detailed study of Raman polarized spectra in betaine arsenate (BA) is pre sented. The results obtained give a significant contribution to the clarifi cation of some controversial aspects of the critical behavior in BA. The te mperature evolution of the hydrogen bonds stretching modes shows that diffe rent types of hydrogen bonds present different kinds of critical behavior. It also provides evidence of a proton ordering in the shorter hydrogen bond , i.e., along the c direction, at the para-ferroelectric phase transition. An order-disorder mechanism in the low-temperature region manifests itself by the progressive softening of the dynamical central peak observed in the tow-frequency spectra. Futhermore, a hint of a short-range ordering below 1 50 K was found which is related to the existence of polar clusters even in the paraelectric phase.