Trigonal Boracites — A New Type of Ferroelectric and Ferromagnetoelectric that Allows no 180° Electric Polarization Reversal

Abstract

Abstract The space group of the trigonal phase of the FeCl, FeBr, FeI, CoCl, and ZnCl boracites is established as C . Ferroelectricity is evidenced by domain switching. High coercive fields at room temperature (up to 800 kV/cm) are found. Polarization‐microscopical studies show that the polarization vector is not reversible by 180° but can jump only from, e.g., [111] to [111], [111], or [111], etc., in accord with crystal structure and group theoretical symmetry considerations. Two twinning “laws” are observed: a) head—head (tail—tail) domains with {110} cub as composition plane and b) head—tail domains with {100} cub as composition plane. The domain structure and the relevant twinning operations are discussed. The onset of ferromagnetism at low temperature in FeCl, FeBr, FeI, and CoCl boracites leads to ferromagnetoelectricity. In the case of CoCl boracite the ferromagentoelectric point group is m.