Effect of substrate clamping on evolution of properties in homovalent and heterovalent relaxor thin films

Relaxor ferroelectrics, well known for their large dielectric and piezoelectric response, are attracting growing interest in thin-film form driven by a need for miniaturized devices. Fundamental understanding and control of the performance of relaxors as thin films, however, is underdeveloped relative to studies of bulk versions. One obvious challenge is substrate clamping, which reduces the dielectric and piezoelectric response, yet systematic comparisons of the effect of substrate clamping on different relaxor materials is missing. Here, the impact of epitaxial constraint on the relaxor behavior and properties of both homovalent ${\mathrm{BaZr}}_0.5{\mathrm{Ti}}_0.5{\mathrm{O}}₃$ and heterovalent $\mathrm{Pb}{\mathrm{Mg}}_1/3{\mathrm{Nb}}_2/3{\mathrm{O}}₃$ relaxors is studied in (001)-, (011)-, and (111)-oriented thin films. While the different orientations of $\mathrm{Pb}{\mathrm{Mg}}_1/3{\mathrm{Nb}}_2/3{\mathrm{O}}₃$ heterostructures show little variation in dielectric permittivity, a strong orientational effect is observed in ${\mathrm{BaZr}}_0.5{\mathrm{Ti}}_0.5{\mathrm{O}}₃$ heterostructures, with (111)-oriented films exhibiting a 67% higher dielectric permittivity than (001)-oriented films, a difference that is attributed to how substrate clamping along different crystallographic axes interacts with the polar-structure nucleation and growth. Measurements of dielectric anisotropy in (001)-oriented films confirm the trend---with $\mathrm{Pb}{\mathrm{Mg}}_1/3{\mathrm{Nb}}_2/3{\mathrm{O}}₃$ heterostructures exhibiting just half as much anisotropy in dielectric permittivity than ${\mathrm{BaZr}}_0.5{\mathrm{Ti}}_0.5{\mathrm{O}}₃$ heterostructures. These results indicate a higher susceptibility to substrate clamping in homovalent relaxors as compared to heterovalent relaxors, implying that the random fields in the heterovalent relaxors could be advantageous for materials in such geometries and applications.