Detwinning Ferroelectric Crystals

A C H of the ferroelectric 1 crystals so far discovered has a structure which closely approaches a more symmetrical structure into which it transforms at the Curie temperature. In all of them, the deviation from the more symmetrical structure is so slight (Table I) t h a t the application of mechanical stress or electric field can produce a shift from one orientation of the lower symmetry structure to another. Since, in crystals grown from the melt, such as barium titanate, inhomogeneous mechanical stresses resulting from inhomogeneous cooling or differential thermal contraction of the surrounding flux material are present in the crystals as they pass through the Curie temperature, these crystals commonly comprise regions of two or more orientations of the lower-symmetry structure, symmetrically related. They are, in other words, twinned. In this condition the electrically polar direction differs in orientation from one individual of the twin to another 2 . Since it is frequently desirable to have the polar direction oriented uniformly throughout the crystal, it is of interest to determine under what conditions this state can be achieved. It is not possible in all crystals. The discussion in this paper will be confined to barium titanate because more experimental data are available for this crystal, b u t it is probable that similar considerations are applicable to the other ferroelectric crystals. T h e process of causing the polar axis in a ferroelectric crystal to have the same orientation throughout the crystal has been called "poling." It is the process of detwinning the crystal.