The existence of polar nanoregions (PNRs) endows relaxor ferroelectrics with peculiar behaviors, which is explained using the compositional fluctuation theory (CFT). Here, by designing relaxor ferroelectric films with the same configurational entropy ($S_{\text{config}}$), namely ${\mathrm{ATiO}}_3$ and ${\mathrm{PbBO}}_3$, we show that the compositional heterogeneity is not a prerequisite for PNR formation. Transmission electron microscope experiments show that PNRs exist in both cubic films, and only the ${\mathrm{PbBO}}_3$ film shows remarkable compositional fluctuation, i.e., the formation of PNRs in ${\mathrm{ATiO}}_3$ is not related with the CFT. First-principles calculations indicate that while the mixing enthalpies of element pairs in ${\mathrm{ATiO}}_3$ are generally close to 0, resulting in a nearly homogeneous compositional distribution, those of some element pairs in ${\mathrm{PbBO}}_3$ are large negative values, indicating that the compositional fluctuation is rooted in the large negative mixing enthalpy. This Letter offers a new perspective on the CFT in relaxor ferroelectrics.

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• Monolayer films
• Perovskites
• Relaxor ferroelectrics
• First-principles calculations
• Transmission electron microscopy