Piezoelectric Resonance in KH2PO4 Type Crystals Revisited
  Within the framework of proton ordering model with taking into account the piezoelectric interaction with the shear strain $\varepsilon_6$, a dynamic dielectric response of KD2PO4 type ferroelectrics is considered. An expression for the piezoelectric resonance frequencies of the rectangular thin plate of the crystal cut in the (001) plane is obtained. 
R. R. Levitskii, I. R. Zachek, A. P. Moina, A. S. Vdovych
cond-mat.mtrl-sci