Gyrotropy: an implicit form of ferroicity

Some optically active ferroic crystals (a few ferroelectrics and one ferroelastic) are known to exhibit reversal of the sign of spontaneous optical rotation in certain directions when a suitable electric field or uniaxial stress is applied. This phenomenon, to be called gyrotropy, is examined in the framework of Aizu's formalism of ferroicity. The case of dicalcium strontium propionate, Ca₂Sr(C₃H₅O₂)₆, a ferroelastic-ferroelectric, is analysed to illustrate a simple procedure for enumerating gyrotropic orientation states. The 18 'pure' gyrotropic species of crystals are listed: these are those species for which gyrotropy is not accompanied by primary ferroicity. Ni-Cl boracite, BaMnF₄, RbNO₃, and some related crystals, are suggested as new potential gyrotropics. The first two of these are ferromagnetic gyrotropics, of which no examples are known so far.