Peak of a gyration tensor component of boracite crystals

According to HAUP measurements, two ferroelectric boracites, Fe3B7O13I and Cu3B7O13Cl, manifest peaks of a gyration component g11 at each Curie point. These phenomena are entirely unintelligible from the usual concepts of the origin of ferroelectricity. Besides, the electrogyration effects of both crystals and also quite different. As has already been determined boracite crystals exhibit characteristic features of being improper ferroelectric. A phenomenological theory of improper ferroelectricity developed by Kobayashi is applied to these anomalous phenomena, and qualitative interpretations are successfully obtained.