946 



THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



Table I 



Substance 



Barium titanate 



Potassium 

 niobate 



Rochelle Salt 



Potassium dihy- 

 drogen phos- 

 phate 



Stable Structure at lower 

 temperature 



Tetragonal 

 c = 4.04, a 



4.00 



Orthorhombic 



a = 5.70, b = 5.74, 

 c = 3.98 equiva- 

 lent to special case 

 of monoclinic in 

 which a = c = 

 4.04, b = 3.98, 

 /3 = 90°, 20' dz 5' 



Monoclinic 



Orthorhombic 



Closely allied more sym- 

 metrical structure and 

 the Curie Temperature 

 above which it is the 

 stable structure 



Cubic, 00= 4.00 

 Tc = ca. 120°C 



Cubic, ao = 4.04 

 Tc = ca. 435°C 



Orthorhombic 

 Tc = ca. 24°C 



Tetragonal 



Tc = ca. -152°C 



Difference 



1% of the length of 

 the c axis 



1.5% of the length of 

 the c axis -f a shear 

 angle of about 20' 



A shear angle of about 

 3' 



Small shear 



Fig. 1 — A. Crystal a as received from the melt. Edges at 45° to polarization direc- 

 tions of crossed nicols. Dimensions of surface: .2 x .2 mm. 



B. Crystal b as received from the melt. Edges at 45** to polarization directions of 

 crossed nicols. Dimensions of surface: .15 x .2 mm. 



With barium titanate as with quartz, the activation energy of this change 

 can be reduced to zero by heating the crystal through a polymorphic transi- 

 tion above which its symmetry is such that the twinning can no longer 

 exist. It is then cooled through the transition under the influence of the 

 applied stress which favors one of the possible twin-orientations. 



