Determination of local high-frequency dielectric function during the cubic-to-tetragonal phase transformation in barium
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Determination of local high-frequency dielectric function during the cubic-to-tetragonal phase transformation in barium titanate Kalpana S. Katti, Maoxu Qian, and Mehmet Sarikaya Department of Materials Science and Engineering, University of Washington, Box 352120, Seattle, Washington 98195
Masuru Miyayama Advanced Materials Department, Research Center for Advanced Science and Technology, University of Tokyo, Tokyo 153, Japan (Received 10 April 1996; accepted 14 November 1996)
Transmission electron energy loss spectroscopy was used to obtain local dielectric properties in barium titanate. The high frequency dielectric function of the material was studied dynamically during the cubic-to-tetragonal (c ! t) phase transformation in conjunction with the effect of a small amount (0.9%) of donor dopant (niobium). In order to obtain the local dielectric function during the phase transformation, Kramers–Kronig relations were applied to the energy loss measurements. The optical excitations in the energy loss spectra were consistent with band structure results from the literature. The Re (1ye), real part of the inverse dielectric function, obtained from the energy loss data indicated a change at the phase transformation. Specifically, a broadening of the valence plasmon excitation is observed which is attributed to the order-disorder nature of the t ! c transformation. A 0.4 eV shift in the volume plasmon was observed in the Nb-doped sample in all regions (within grains as well as at grain boundaries), indicating a uniform incorporation of the dopant in the lattice. In this paper, the changes in the dielectric function, such as shifts in collective excitations, are attributed to a large contribution from loosely bound Nb electrons. Furthermore, it is demonstrated that it is possible to obtain local (ø10 nm) physical property of a complex material dynamically at relatively high temperature.
I. INTRODUCTION
BaTiO3 is a prototype of ferroelectrics having the simplest form of the perovskite structures; it is a precursor of several modern day electroceramics, such as capacitors and positive temperature coefficient of resistivity (PTCR) thermistors.1–5 There has recently been an increased interest in the development of perovskite ferroelectrics for electro-optic and other electronic applications.1,6,7 Besides the issues related to the processing strategies of depositing thin and uniform epitaxial films on suitable substrates, the successful applications of these materials depend on the understanding of their optical and dielectric properties, and their tailoring for specific applications that highly depend on dopants. For example, many properties of perovskites, like other polycrystalline materials, vary depending on the size and shape of grains, defects, and substructures, and grain boundary structures and composition. For example, the addition of a small amount of donor dopant, e.g., Nb, to BaTiO3 significantly changes its physical properties, such as resistivity characteristics, a
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