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

The space group of the trigonal phase of the Fe-Cl, Fe-Br, Fe-I, Co-Cl, and Zn-Cl boracites is established as C63v. Ferroelectricity is evidenced by domain switching. High coercive fields at room temp. (up to 8000 kV/cm) are found. Polarization-microscopic 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 compn. plane and (b) head-tail domains with {100}cub as compn. plane. The domain structure and the relevant twinning operations are discussed. The onset of ferromagnetism at low temp. in Fe-Cl, Fe-Br, Fe-I, and Co-Cl boracites leads to ferromagnetoelectricity. In the case of Co-Cl boracite, the ferromagnetoelec. point group is m