Possible Species of “Ferroelastic” Crystals and of Simultaneously Ferroelectric and Ferroelastic Crystals

It is convenient to grasp crystals of a kind as “ferroelastic” crystals. They are defined analogously to “ferroelectric” crystals, mechanical strain and stress in the former corresponding to electric polarization and field in the latter respectively. The order of magnitude of the coercive stress for ferroelastic state-shifts which corresponds to the coercive field for ferroelectric state-shifts is expected, on the average, to be 10 2 newt·cm -2 . A determination is made of all theoretically possible species of ferroelastic crystals and the number of states and ferroelectricity or nonferroelctricity in each of these species. It is found that there are 94 of these ferroelastic species and that 42 of them are simultaneously ferroelectric. Incidentally, an alteration is made of the hitherto used symbols for the 88 species of ferroeletric crystals.