Determination of the Kind for Actual Ferroelectrics, Together with Examination of the Characters of Their Paraelectric Phase Transformations

It was shown that the ferroelectrics are divisible into 30 kinds in accordance with their point groups. Bravais lattices, and types of state transition; 19 of these 30 kinds are regular and the remaining 11 are irregular. In this paper, the determination of the kind is attempted for the actual ferroelectrics discovered up to now. Some useful methods for this purpose are explained. It is confirmed that every actual ferroelectric does belong to one of the 30 kinds. On the other hand, it is seen that, at the present stage, not all of the 30 kinds have actual ferroelectrics belonging to them. Incidentally, an examination is made of the characters of the paraelectric phase transformations of actual ferroelectrics. It is found that only the barium titanate family and the methylammonium aluminum sulfate dodecahydrate family (and possibly the ammonium cadmium sulfate family) have a complex phase transformation, and all the others a primitive phase transformation. (The ferroelectrics having no paraelectric phase transformation are excluded.) Among the kinds not occupied by any actual ferroelectric, the kinds $i1\mathrm{}$-I, $i1\mathrm{}$-II, and $\mathrm{im}$ are singular, because ferroelectrics belonging to them, if any, should change to paraelectrics of polar point groups by a primitive phase transformation. A suggestion is made of a way of discovering such ferroelectrics. The present work also serves for finding gyroelectrics and hypergyroelectrics among known ferroelectrics.