A Novel Technique for Determining Local Dielectric Function During Ferroelectric To Paraelectric Phase Transformation in
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ABSTRACT In this work a transmission electron microscopy (TEM) technique was used in obtaining local dielectric properties calculated from optical parameters for dynamic investigation of the effect of cubic to tetragonal phase transformation in barium titanate. In order to obtain in situ local dielectric during phase transformation, Kramers-Kronig relations were applied using the transmission electron energy loss (EELS) measurements. The optical excitations in the EELS spectra were consistent with the band structure results. The Re (1/) (real part of the dielectric function) obtained from the energy loss data indicated a change at the phase transformation. A broadening of the valence plasmon excitation suggested an order-disorder nature to the cubic to tetragonal transformation. In situ electron energy loss near edge structure (ELNES) studies from 500-700 eV energy range near the O-K edge exhibited a pre-edge feature that is associated with the Ti-L, edge which further indicates an order-disorder nature to the phase transformation. The significance of the results is discussed. INTRODUCTION Barium titanate is considered to be a prototype of perovskite ferroelectrics.' The classical ferroelectrics such as potassium dihydrogen phosphate (KDP) and others have complex hydrogen bonded structures. The perovskite structure of barium titanate is the simplest of the ferroelectric structures. X-ray absorption near edge structure (XANES) experiments have revealed that preedge feature, associated with principle edges of transition metal ions, which are indicative of the broken centrosymmetry in the crystal since they are associated with forbidden quadrupole transitions.3 4' Thus some order-disorder nature to the classically displacive type' phase transformations in perovskites was suggested. In the displacive transformation the Ti4 ion is regularly displaced from the (1/4,1/4,1/4) position in the cubic lattice towards either of the directions throughout a given domain hence assuming a tetragonal lattice. In the order-disorder transformation, the regions containing this displacement is small (nanoscale) and random throughout the tetragonal phase. EELS is a useful technique for probing the electronic structure of materials.5 Recent years have seen the emergence of the transmission EELS technique as a means to understand the electronic structure of materials. 6'9 Also the band structure of barium titanate is a subject of a detailed investigation in both experimental and theoretical research from the points of view of understanding the ferroelectric properties and of furthering its properties."" 4 The theoretical evolution of the electronic structure through phase transformation in barium titanate has not been well understood yet. The dielectric function profile displayed dynamically through phase transformations can provide a valuable insight into the local properties of the material. The energy loss near edge fine structure (ELNES) of principle edges in the EELS spectra also hold useful information of the local surroundings of th
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