138 BELL S YSTEM TECH NIC A L JOURNA L 



not considered here. The spontaneous ^ae constant affects the shear con- 

 stant ^66 for crystals rotated about the A' axis and could be detected experi- 

 mentally. No experimental values have been obtained. 



The effects of spontaneous polarization in the second equation of (164) 

 are of two sorts. For an unplated crystal, a spontaneous voltage is gen- 

 erated on the surface, which, however, quickly leaks off due to the surface 

 and volume leakage of the crystal. The other effects are that the spon- 

 taneous polarization introduces new piezoelectric constants through the 

 tensor Qkfmn , changes the dielectric constants through the tensor Omno and 

 introduces a stress effect on the piezoelectric constants through the tensor 

 Mkfmqr ■ Siuce piezoelectric constants are not as accurately measured as 

 elastic constants, the first effect has not been observed. The additional 

 piezoelectric constants introduced by the tensor Qkfmn are shown by equa- 

 tion (180) 







g2, g26 (180) 



Since the only constants for the Rochelle salt class, the orthorhombic 

 bisphenoidal, are gu , g2b , gse , this shows that between the two Curie points 

 the crystal becomes monoclinic sphenoidal, with the A' axis being the 

 binary axis. The added constants are, however, so small that the accuracy 

 of measurement is not sufficient to evaluate them. From the expansion 

 measurements of equation (172) and the spontaneous polarization values, 

 three of them should have maximum values of 



gn = -6.6 X 10"^ gu = +1.3 X 10"'; gn = -1.8 X 10"' (181) 



These amount to only 6 per cent of the constant gu , and hence they are 

 not easily evaluated from piezoelectric measurements. 



The effect of the tensor Omuo is to introduce a spontaneous dielectric 

 constant €23 between the Curie temperatures so that the dielectric tensor 

 becomes 



en, , 



0, e,,, 623 (182) 



, €23 , f33 



As discussed at length by Mueller'"* this introduces a spontaneous bire- 

 fringence for light passing through the crystal along the A', 1' and Z axes 

 which adds to the birefringence already present. 



« "Proi)crtics of Rochcilc Salt I and IV," Phvs. Rev. 47, 175 (1935); 58, 805 November 1, 

 1940. 



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