Detwinning Ferroelectric Crystals 



By ELIZABETH A. WOOD 



Unstrained single crystals of barium titanate can be detwinned under the 

 influence of an electric field at elevated temperature, but strained crystals can- 

 not. It seems probable that this is also true of crystals in a polycrystalline body 

 such as a ceramic. 



EACH of the ferroelectric^ 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) that 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 re- 

 sulting 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^. 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, but it is probable that 

 similar considerations are applicable to the other ferroelectric crystals. 



The 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. As C. J. Davisson and others pointed out 

 in connection with the problem of detwinning quartz crystals during World 

 War II, if the crystal is subjected to a stress which will be lessened if the 

 "misoriented" regions change to the desired orientation and if the activa- 

 tion energy of the change is not too great, the crystal will be detwinned. 



^ Ferroelectric crystals are those crj'stals which exhibit, with respect to an electric 

 field, most of the phenomena exhibited by ferromagnetic crystals with respect to a mag- 

 netic field, such as spontaneous polarization, domain structure, hysteresis of response to 

 an alternating field and a Curie temperature above which these unusual characteristics 

 are not present. 



' By the ferromagnetic analogy each twin individual is called a ferroelectric "domain." 



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