Orientation-switching transition and ferroelectricity in betaine arsenate

This paper reports a detailed investigation of the low frequency dielectric relaxation of betaine arsenate near the ferroelectric phase transition. The dielectric relaxation data are complemented with polarized infrared reflectivity data taken at low temperatures. The reported results allow us to identify several low frequency modes that clarify the complex behaviour of the dielectric response near the Curie temperature Tc2∼120 K. It is suggested that the important slow dynamics observed is linked to the reorientation of the betaine molecular group. The roles of the different molecular units in the structural changes are briefly discussed and a new and more complex phase transition sequence is proposed.