Properties of Rochelle Salt. IV

A new class of effects in crystals, called morphic effects, is considered. They arise when elastic strains alter the symmetry of the structure. This necessitates the introduction of new coefficients, proportional to the acting strains, into the matrix of the coefficients which characterize the linear effects. The new coefficients give rise to a series of new quadratic effects in crystal physics. These morphic effects play a role in the photoelasticity of cubic crystals. They are especially large in the crystals of Rochelle salt, where they are responsible for the so-called Kerr effects and a new type of irreversible piezoelectric effects. The existence of this new effect has been verified. It creates irreversible normal strains when the crystal is polarized in the $a$ direction, and spontaneous contractions occur when the crystal is cooled to below the Curie point. The spontaneous normal strains are responsible for the anomalous thermal expansion of Rochelle salt and for a series of other anomalies. It is suggested that these strains may also be the cause for the existence of the lower Curie point.