| Summary: | Structure, phonon, and energy storage density in Sr<sup>2+</sup>-substituted lead-free ferroelectric Ba<sub>1−x</sub>Sr<sub>x</sub>TiO<sub>3</sub> (BST<sub>x</sub>) for compositions x = 0.1, 0.3, and 0.7 were investigated using X-ray diffraction, Raman, and ferroelectric polarization measurements as a function of temperature. The samples were tetragonal for x = 0.1 with a large c/a ratio. The tetragonal anisotropy was decreased upon increasing x and transforming to cubic for x = 0.7. The changes in structural and ferroelectric properties were found to be related to the c/a ratios. The temperature-dependent phonon spectroscopy results indicated a decrease in tetragonal–cubic phase transition temperature, Tc, upon increasing x due to a reduction in the lattice anisotropy. The intensity of ~303 cm<sup>−1</sup> E(TO2) mode decreased gradually with temperature and finally disappeared around the tetragonal ferroelectric to cubic paraelectric phase at about 100 ℃ and 40 ℃ for x = 0.1 and 0.3, respectively. A gradual reduction in the band gap E<sub>g</sub> of BST<sub>x</sub> with x was evident from the analysis of UV-visible absorption spectra. The energy storage density (U<sub>dis</sub>) of the ferroelectric capacitors for x = 0.7 was ~0.20 J/cm<sup>3</sup> with an energy storage efficiency of ~88% at an applied electric field of 104.6 kV/cm. Nearly room temperature transition temperatures T<sub>C</sub> and reasonably fair energy storage density of the BST<sub>x</sub> capacitors were found.
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