Grain-size-induced ferroelectricity in NaNbO₃

The influence of a material's length scale on its functional properties has been reported for many ferroics, where a decrease in the dielectric, ferroelectric, and piezoelectric properties is frequently observed when the size is reduced below the micrometer range. Here we demonstrate that in NaNbO₃, in contrast to general expectations, a long-range ferroelectric order is induced when decreasing the size. A series of sintered, polycrystalline, NaNbO₃ samples with different grain sizes was prepared and analyzed using differential scanning calorimetry, dielectric measurements, and ²³Na 3QMAS nuclear magnetic resonance (NMR). A size-induced phase transition into the ferroelectric polymorph was observed when the grain size decreased below 0.27 μm. The different polymorphs were distinguished based on the local symmetry of the Na(1) sodium site. NMR was also used to determine the relative amounts of the polymorphs in the samples. The observed size-induced phase transition is attributed to the existence of intragranular stresses, induced by the decreased compensation of the ferroelastic energy during the formation of non-180° domain walls when decreasing the grain size and the large anisotropy of the thermal expansion. The results demonstrate the unique possibility of stabilizing ferroelectricity by reducing the grain size, which was not observed in other ferroic systems.