Structural and Vibrational Properties of Ferroelectric Pb 1-x Sr x TiO 3 Thin Films and Powders
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Structural and Vibrational Properties of Ferroelectric Pb1-xSrxTiO3 Thin Films and Powders M. Jain, A. Savvinov, P.S. Dobal, S.B. Majumder, R.S. Katiyar, and A.S. Bhalla* Department of Physics, University of Puerto Rico, San Juan, PR 00931, USA * Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA
ABSTRACT In this work we present the structural, and vibrational properties of ferroelectric Pb1-xSrxTiO3 (PST). Thin films of PST were prepared by using sol-gel technique for various compositions with x values ranging from 0.0-1.0. Respective compositions were also prepared in ceramic and powder forms using sol-gel and solid-state reaction methods. X-ray diffraction was used for the structural characterization of these materials. Raman spectroscopy was utilized to study the phases and lattice vibrational modes, especially the soft mode in PST compositions. The temperature dependence of the soft mode frequency for different PST compositions revealed that the phase transition temperatures increased with increasing Pb contents in PST system. Ferroelectric properties of the films were correlated with the substitution-induced changes in the material.
INTRODUCTION Lead titanate (PbTiO3) belongs to perovskite structure with a simple ferroelectric to paraelectric cubic transition around 490oC. On the other hand, strontium titanate SrTiO3 is a quantum paraelectric [1] whose dielectric constant continues to increase upon cooling down to about 4K, below which it levels off due to zero-point quantum fluctuations. The Curie temperature of lead strontium titanate (Pb1-xSrxTiO3) was found to decrease as the Sr contents increase in the system [2]. With certain compositions, PST can be prepared with the Curie temperature just below the room temperature so that it is paraelectric with high dielectric constant around room temperature. Recently these materials have shown potential as capacitors of ultra large-scale integration DRAM cell and for tunable dielectric devices [3,4]. Also the dielectric loss in some of the PST compositions is relatively smaller as compared to the barium strontium titanate (BST) compositions. The studies are therefore desirable to understand the intrinsic behavior of the loss mechanism and its relation to the soft modes in such systems. Raman scattering has proven to be a valuable technique since it is a non-destructive characterization technique, sensitive to structural and dynamical properties. The soft mode behavior is directly linked to the ferroelectric properties and phase transitions occurring in a material. The phase transitions in PbTiO3 and SrTiO3 (observed in Raman scattering) can be correlated with the softening of the lowest TO phonon mode with temperature or pressure [5-7]. Raman experiments have been performed on PT and PST systems [5,8,9]. However, there is no systematic temperature-dependent Raman studies on the PST compositions. Low frequency Raman studies on the lead based materials was found to influence the Curie temperature of lead titanat
