In situcharacterization of the ferroelectric transition inBaTiO3by EELS and comparison withab initiomethods

Changes in the dielectric properties during the ferroelectric transition of commercial $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ powders were determined in situ, by analyzing the low-energy-loss region of electron energy loss spectroscopy (EELS) spectra in a transmission electron microscope, at room temperature (ferroelectric phase) and $150\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ (paraelectric phase). A comparison of experimental EELS spectra and ab initio density-functional theory calculations (WIEN2K code) within the generalized gradient approximation (GGA) is presented. A characteristic peak around $11\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ appears in the imaginary part of the dielectric function in paraelectric phase, which is absent in ferroelectric phase. The origin of the characteristic peak is analyzed by means of energy band structure calculations.