Structural, dielectric and ferroelectric studies of thermally stable and efficient energy storage ceramic materials: (Na0.5-xKxBi0.5-xLax)TiO3

The structural, dielectric and ferroelectric properties of lead-free (Na0.5-xKxBi0.5-xLax)TiO3 (0 <= x <= 0.12) powders synthesized by modified sol-gel method was investigated. Rietveld refinement of synchrotron x-ray diffraction data confirms single phase rhombohedral crystal structure with R3c space group for all the compositions and anti-phase (a(-)a(-)a(-)) octahedral tilting angle decreased with increase in composition. Homogeneity and elemental proportions were confirmed by Energy dispersive x-ray spectrometry. The temperature-dependent dielectric study has shown two diffuse type of dielectric anomaly for all the samples, due to A-site disorder in the lattice, which has been assigned to two-phase transitions: ferroelectric to anti-ferroelectric and anti-ferroelectric to the paraelectric phase transition. Thermal stability range of dielectric constant increases from similar to 100 to 220 degrees C as a function of composition. Stable dielectric constant first increases from 1557 +/- 10% for x = 0 compositions and highest for x = 0.06 sample with epsilon(mid) similar to 2508 +/- 10% of the temperature range similar to 180-340 degrees C, and after that decreases to 1608 +/- 10% for x = 0.12, but remain higher than the x = 0 composition. Ferroelectric measurements have shown monotonously decreasing coercive field as a function of the composition due to decrease in the average grain size, confirmed by microstructural studies using field emission scanning electron microscope. An exponential increase in the energy storage efficiency from similar to 17% to 87% as a function of composition have also observed. These types of materials, with the high stable dielectric constant (epsilon(r)) and low loss (tan delta), have a vast scope in the field of the thermally stable dielectric constant materials and energy storage applications.