Structural and magnetic phase transitions in Fe-I boracite, Fe₃B₇O₁₃I

The structural and magnetic phase transitions of Fe-I boracite have been studied by means of optical examinations of the domain structure and by measurements of the spontaneous birefringence using polarized-light microscopy. These phase transitions have also been characterized by dielectric and spontaneous polarization studies, realized on ferroelastic/ferroelectric single-domain states, which were obtained by electric field poling along 〈100〉_cub and 〈111〉_cub directions, with simultaneous visual control of the domain state. In the trigonal 3ml′ phase, an electric field E∥〈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 polarity gave rise to domains with the three other preferential directions of P_s, non-collinear with E, and to a polarization of P_s/3, indicating the non-180°-reversibility of P_s along 〈111〉_cub The magnetic phase transition 3ml′ ↔ m resulted in anomalies both in the dielectric permittivity (up to 2 MHz) and in the spontaneous 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.