Correlation between enhanced lattice distortion and volume fraction of polar nanoregions in quenched Na₁/₂Bi₁/₂TiO₃–BaTiO₃ ceramics

Quenching has been established as a viable method to increase the depolarization temperature in (100-x) Na₁/₂Bi₁/₂TiO₃–xBaTiO₃ (NBT–xBT). The proposed hypothesis of a stabilized ferroelectric order would entail changes in the polarized volume. To this end, air-quenched and furnace cooled samples of four compositions of NBT–xBT with x = 3, 6, 9, and 12 mol. % BT were studied. Upon quenching, all the compositions demonstrate an increase in the ferroelectric to relaxor transition temperature, TF-R, by 23–44 °C and enhanced lattice distortion. Resonance frequency damping analysis was utilized to measure Young's modulus in the temperature range of 25 °C to 800 °C and to estimate the volume fraction of polar nanoregions using a composite model. Quenching leads to an 8% decrease in Young's modulus, but to an increase in the volume fraction of polar nanoregions by 12% at 300 °C for NBT-6BT. Transmission electron microscopy investigations of quenched NBT-6BT reveal a combination of lamellar domains and more homogenous areas with nanometer-sized domains. The existence of lamellar domains in quenched morphotropic phase boundary compositions together with enhanced lattice distortion and a decrease in dielectric frequency dispersion substantiate the premise of a stabilized ferroelectric order.