Phase transition sequence of betaine arsenate investigated by infrared reflectivity

The ac-polarized infrared reflectivity spectra of betaine ortho-arsenic acid single crystals have been measured throughout the ferroelastic-ferroelectric phase transition observed near Tc2 = 120 K. The monoclinic symmetry of the system was explicitly taken into account and a three-polarization technique used for the first time to investigate in detail the critical behaviour of the Bu polar phonons. The results show that near the THz frequency range, the usual approximation of considering the system as pseudo-orthorhombic fails, giving rise to errors in the determination of the optical parameters. The obtained spectroscopic data indicate that the phase diagram and the physical mechanism involved in the stabilization of ferroelectricity in betaine arsenate (BA) are more complex than the prevalent view of a simple ordering process in bistable hydrogen bonds.