STRUCTURAL AND MAGNETIC PHASE-TRANSITIONS IN FE-I BORACITE, FE3B7O13I

The structural and magnetic phase transitions of Fe-I boracite have be en studied by means of optical examinations of the domain structure an d by measurements of the spontaneous birefringence using polarized-lig ht microscopy. These phase transitions have also been characterized by dielectric and spontaneous polarization studies, realized on ferroela stic/ferroelectric single-domain states, which were obtained by electr ic field poling along (100)(cub) and (111)(cub) directions, with simul taneous visual control of the domain state. In the trigonal 3m1' phase , an electric field E parallel to(111)(cub) of one polarity produced a monodomain with the spontaneous polarization P-s and the optical axis perpendicular to the (111)(cub) plane, while that of the reversed pol arity gave rise to domains with the three other preferential direction s of P-s, non-collinear with E, and to a polarization of P-s/3, indica ting the non-180 degrees-reversibility of P-s along (111)(cub). The ma gnetic phase transition 3m1' reversible arrow m resulted in anomalies both in the dielectric permittivity (up to 2 MHz) and in the spontaneo us polarization at the Curie temperature T-C = 31 K. This suggests an interaction between the magnetic moment and the electric polarization, consistent with the ferroelectric/ferroelastic/ferromagnetic coupling effect previously found for the m phase.