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.

The theoretical calculation of orientation dependence of the electromechanical coupling factor for the thicknessextensional mode, k,, in KNbO3 crystal has shown that it is as high as 69% for the 49.5" rotated X-cut about the Y-axis, which is the highest among known piezoelectricS. To experimentally confirm this, the i m m c e responses of the thickness-extemional mode vibrators are measured and the related constants at^ evaluated. The measured coupling factor is as high as 70%. The strain vs electric-field behaviors of the pseudocubic (001) cut, which is close to the 49.5" rotated X a t , are measured. The strain vs electriofield curves exhibit large hysteresis in a high electric field range for as-cut single domain crystals, whereas for etched single domain crystals they exhibit good linearity even in the high electric field range. Similar experiments are performed for multidomain (001) cut crystals poled along the