  The structural, dielectric and ferroelectric properties of lead-free
(Na0.5-xKxBi0.5-xLax)TiO3 powders synthesized by sol-gel self-combustion method
were investigated. Rietveld refinement of Synchrotron x-ray diffraction data
confirms pure single phase rhombohedral crystal structure with R3c space group
for all the compositions and anti-phase octahedral tilting angle decreased with
increase in composition x. 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-has
transitions: ferroelectric to anti-ferroelectric and anti-ferroelectric to the
paraelectric phase transition. The transition temperature of these phase
transitions is found to decrease as a function of composition. Thermal
stability range of dielectric constant increases from ~100C to 220C as a
function of composition. Stable dielectric constant first increases, from 1557
10 % for parent compound, with the composition, highest for 6 % composition
with emid ~ 2508 10 % for the temperature range ~180 C to 340C and after that
decreases to 1608 10 % for 12 % but remain higher than the parent compound
Na0.5Bi0.5TiO3. Ferroelectric measurements have shown monotonously decreasing
coercive field as a function of the composition due to a decrease in grain
size, confirmed by microstructural studies using Field Emission Scanning
Electron Microscope. Exponential increases in the energy storage efficiency
from ~ 17 % to 87 % as a function of composition have also observed. These
types of materials, with stable high dielectric constant and low tan delta,
have a vast scope in the field of the thermally stable dielectric constant
materials and energy storage applications.
