Studies on Electrical Energy Storage Density in Lead-Free Ferroelectric Ba_1-x Sr_xTiO₃ (x = 0.1, 0.3, and 0.7) Capacitors

Structure, phonon, and energy storage density were studied in Sr2+-substituted lead-free ferroelectric Ba1-x SrxTiO3 (BSTx) (x = 0.1, 0.3, and 0.7) samples, prepared using solid state reaction techniques, utilizing X-ray diffraction, Raman, and ferroelectric polarization measurement techniques. The system is tetragonal for x = 0.1 with large c/a ratio at room temperature. The tetragonal anisotropy decreases with increasing x, and becoming cubic for x > 0.3. All these structural properties change due to change in the c/a ratio upon rising temperature. Temperature dependent phonon spectroscopic results (80-500 K) indicate reduction in tetragonal to cubic phase transition temperature upon increasing x due to reduction in tetragonal anisotropy (c/a). Overdamping of ~90 cm-1 E-soft phonon mode and a dramatic decrease in the intensity of ~160 cm-1 A-mode were observed at around the tetragonal ferroelectric to cubic paraelectric phase. Using UV-Vis optical absorption spectra, the optical band gaps of these compounds were calculated as 1.73, 1.7, and 1.68 eV as a function of increasing the dopant concentration (x). The energy storage densities of these electrically polled BSTx capacitors were estimated using the polarization hysteresis loops. For x = 0.7, a large energy storage density (Ure) of 20 Joule/cm3 with the efficiency of 57 % was estimated at applied electrical field of 88 kV/cm. Close to Tc large dielectric constant and high energy density values of BSTx capacitors indicate its possible application in high energy storage and pulse power devices. These results will be presented in detail at the meeting.