Observation of hidden order in in-plane ferroelastic superdomains

BaTiO₃ thin films under conditions between structural or ferroelectric phase boundaries exhibit rich ferroelastic superdomain structures that underpin exceptional dielectric and electromechanical responses. Enhanced dielectric tunability has been attributed to coexisting superdomain variants, but the hierarchical organization of domains and the mesoscale domain physics remain challenging to explore experimentally. Using synchrotron hard X-ray nanoprobe diffraction, we have directly visualized the hidden structural order within the domain pattern, which is manifested as a subtle structural modulation, within the superdomain organization. Prior models have included randomly distributed c⁺/c^- domains or purely in-plane ferroelastic domains. Our measurements reveal instead a mesoscale organization of c domains into correlated superdomain networks, reflecting the interaction between ferroelastic and ferroelectric orders. The results provide a pathway to bridge structural hierarchy to functional response, highlighting hidden domain features as a route to produce tunable functionalities beyond those emerging from conventional domain models.