Film Thickness Dependence of Crystal Structure in 100-Oriented Epitaxial Pb(Zr_0.65Ti_0.35)O₃ Films Grown on Single-Crystal Substrates with Different Thermal Expansion Coefficients

100-oriented epitaxial Pb(Zr_0.65Ti_0.35)O₃ films with various film thicknesses from 0.1 to 3 µm were grown on (100)_cSrRuO₃ ∥ (100)SrTiO₃ and (100)_cSrRuO₃ ∥ (100)LaNiO₃ ∥ (001)CaF₂ substrates. The out-of-plane/in-plane lattice parameter ratio of the films on the CaF₂ substrates was larger than that on the SrTiO₃ substrates up to 1.1 µm film thickness, while (90°-α) (α was defined as the internal tilt angle) was almost 0°. Results of analysis of Raman spectra and piezoresponse images suggest that the 1.1-µm-thick film grown on the (100)_cSrRuO₃ ∥ (100)LaNiO₃ ∥ (001)CaF₂ substrate had tetragonal symmetry with a polar-axis orientation. Moreover, the saturation polarization values of the films measured from P–E hysteresis loops correspond to the two P_s values estimated from the thermodynamic theory, assuming the change in the polar direction due to the symmetry change to tetragonal, and from the crystal distortion in tetragonal symmetry. This can be explained by the large compressive stress from the CaF₂ substrate having a large thermal expansion coefficient.