DETWINNING FERROELECTRIC CRYSTALS 951 



(101) plane-normal, but rather, when the non-Hnear scale factor has been 

 taken into account, are all normal to the (100) axis which lies parallel to 

 the film in a top-to-bottom direction. They therefore come from crystal 

 planes bent around this axis of a twinned barium titanate crystal. 



Fig. 6 — Laue photograph of a barium titanate crystal with only two twin-related 

 orientations. 



A section through the reciprocal lattice of a twinned barium titanate 

 crystal is shown in Fig. 8A: that of a bent crystal, in 8B. In the reciprocal 

 lattice of a tetragonal crystal the direction of each point from the origin is 

 the same as the direction of the normal to the set of planes it represents 

 and the distance of each point from the origin is proportional to the recip- 

 rocal of their interplanar spacmg. Since the back reflection Laue photograph 

 shows only orientations of the atomic planes it may be thought of as a 



