Domain structures in $KNbO3$ crystals and their piezoelectric properties

The electromechanical coupling factor for the thickness-extensional mode, $kt,$ in $KNbO3$ crystals has been predicted to be as high as 69% for the 49.5° rotated X-cut about the Y-axis, which is the highest among known piezoelectrics. This paper presents the experimental confirmation of the high coupling factor and elucidates the relationship between the domain structure and the piezoelectric properties in the pseudocubic $(001)pc$ cut, which is close to the maximum $kt$ cut and has a piezoelectric strain constant about 2.9 times that of the Z-cut. Etching of crystals is found to have an outstanding effect on the piezoelectric characteristics. That is, in the case of “as-cut” crystals, the strain versus electric field curves have a large hysteresis and instability due to the occurrence of 60° domains in a high electric field region, whereas etched crystals remain the single domain state and exhibit linear piezoelectric properties even in the high electric field region. It is also demonstrated that the $(001)pc$ cut crystal poled along the $〈001〉pc$ direction has a polar multidomain structure composed of two kinds of 90° domains, not four kinds of equivalent domains, and that it exhibits almost the same piezoelectric properties as those of single-domain crystals.