Effect of Mn Doping on the Structural and Ferroelectric properties of SrBi 2 Ta 2 O 9
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Effect of Mn Doping on the Structural and Ferroelectric properties of SrBi2Ta2O9 T. Sookawee,1 L. Pdungsap,1 P. Winotai,1 R. Suryanarayanan,2,3 V.M. Naik4 and R. Naik2
Department of Chemistry, Faculty of Science, Mahidol University, Bangkok 10400 Thailand Department of Physics and Astronomy, Wayne State University, Detroit, MI, 48201 USA 3 LPCS, UMR 8648, Bât.410, Université Paris-Sud, 91405 Orsay, France 4 Department of Natural Sciences, University of Michigan-Dearborn, Dearborn, MI, 48128 USA 1 2
ABSTRACT SrBi2Ta2O9 (SBT) samples have been prepared by sol-gel technique with varying amount of excess Bi added to the starting composition. The excess Bi profoundly affects the dielectric and ferroelectric properties. The dielectric constant (ε) and remnant polarization increases from 80 and 1.4 µ C/cm2 for SBT with 1 mole % of excess Bi to 120 and 6.5 µ C/cm2 for SBT with 20 mole % of excess Bi. The Raman spectra, scanning electron microscopy images and the x-ray diffraction analysis show an improvement in structural quality of the samples with addition of excess Bi to the starting composition. The effect of Mn doping has also been studied by preparing SrBi2Ta2-xMnxO9 (SBT-Mn) with x = 0 − 0.1. In the case of SBT-Mn samples, the XRD spectra revealed an unknown phase when x > 0.025. However, Raman spectra reveal the retention of Bi layer structure for all Mn compositions, with possibly small distortions. With Mn addition, ε increased to ~ 140 for x < 0.005, but decreased to ≤ 120 when x > 0.015. The value of ε is independent of frequency from 60 to 85 kHz for all compositions of Mn. The effect of Mn addition resulted in a strong decrease in the area of the hysteresis loops. INTRODUCTION The bismuth layer-structured ferroelectric SrBi2Ta2O9 (SBT) because of its low fatigue endurance and low switching voltage has become the most important material for applications in the field of ferroelectric non-volatile random access meomories.1,2 The chemical formula is generally given by Bi2A m-1BmO3 m+3 = [Bi2O2]2+ + [Am-1 BmO 3m+1]2-, where A = Na+, K+, Ba2+, Ca2+, Pb2+, Sr2+, Bi3+and B = Fe3+,Ti4+,Nb5+,Ta5+,W6+ etc. A pseudo-perovskite layer [Am-1BmO 22+ ; m and m-1 are the numbers of 3m+1] is sandwiched between the fluorite type sheets [Bi2O2] oxygen octahedra and pseudo-perovskite units in a pseudo-perovskite layer, respectively.3–5 SBT undergoes a ferroelectric phase transition at about Tc= 608 K. The structural change was previously reported from tetragonal to orthorhombic with a space group Fmmm.4 The detailed crystal structures reveal an orthorhombic structure of SBT, only slightly distorted from tetragonal.6,7 The processing conditions, A- and B-site substitution, size-dependent Raman spectra and the structure–property correlations have been recently reviewed.8 Some substitutional studies of SBT with Ba, Ta, Nb and Ti have been reported.8 Here , we report on the preparation, Raman spectra and dielectric properties of SBT with excess Bi and SBT substituted with Mn. EXPERIMENT The polycrystalline powders of SBT (w
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