Coupled local residual shear and compressive strain in NaNbO₃ ceramics under cooling

Stabilizing lead-free antiferroelectrics at room temperature is key for advancing greener and more efficient energy storage devices. While NaNbO₃ solid solutions hold great promise for high energy density applications, its pure form displays structural instabilities arising from irreversible electric-field induced phase transitions and/or an undesired coexistence with its ferroelectric polymorph. To unravel how mechanical constraints imposed by residual stresses, structural defects, and microstructure disrupt the stability of the NaNbO₃ antiferroelectric state, we used in situ Dark-Field X-ray Microscopy to map local microstructural deformations in a single embedded {100}pc grain. By replicating typical heat treatment conditions, we show that the ferroelectric phase nucleates as a result of the coupled interplay between residual shear and compressive strain distributions that manifest during cooling towards ambient temperature. In addition, the microstrain relaxation behavior indicates that long-range defects preferentially nucleate at the expense of the antiferroelectric phase in regions at sub-micrometer distances from the grain center. Our findings illustrate that adequate temperature control during low temperature sintering, heat treatments, or in operando conditions may be vital in dictating the structure-property relationships of NaNbO₃ ceramics, ensuring their suitability for efficient energy storage applications.