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.