Determination of the State Parameters and Formulation of Spontaneous Strain for Ferroelastics

For a crystal which can become ferroelastic , it is important to know those elements of those linear combinations of elements of mechanical strain tensor each of which, at null stress, is not equal in all the orientation states of the ferroelastic phase and is zero over the whole temperature range in the prototypic phase. These quantities are a kind of state parameters . Also, it is convenient to introduce “spontaneous strain” as a total measure for the `degree' of ferroelasticity. Spontaneous strain should be independent of choice of the rectangular coordinate system, be the same in all the orientation states of the ferroelastic phase, and be zero over the whole temperature range in the prototypic phase. For each of the 94 species of ferroelastics, the set of state parameters is found, its way of changing with each state shift is determined, and the spontaneous strain (satisfying the above conditions) is formulated.