Dependency of direct and inverse transverse piezoelectric properties on composition in self-polarized epitaxial (K_xNa_1-x)NbO₃films grown via a hydrothermal method

(KxNa1-x)NbO3 films that were approximately 3 μm in thickness and with various x values were systematically deposited at 240 C on (100)cSrRuO3//(100)SrTiO3 substrates using a hydrothermal method. Direct and inverse transverse piezoelectric coefficients, e 31,f, near 0 kV cm-1 were approximately -5.0 C m-2 for the as-deposited films with a wide range of x values without any poling treatment. A large difference in e 31,f value was not observed before or after poling treatment upon applying a positive electric field, suggesting that the as-deposited (KxNa1-x)NbO3 films with x = 0.36-0.88 were almost fully in a self-polarized state. (KxNa1-x)NbO3 films with x = 0.36-0.88 without any poling treatment showed a figure of merit for an energy harvester (FOM = e 31,f2/ϵ r) above 0.13, as calculated directly from e 31,f. These FOM values are relatively high compared with the reported ones for other lead-free films that have a perovskite structure.