Phase development in Si modified sol-gel-derived lead titanate
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Sol-gel methods were used to prepare chemically modified lead titanate (PT) powders. A PT alkoxide solution was synthesized and doped with Si (2-12 mole%) or with equimolar amounts (2-12 mole%) of Si and Pb through the addition of Si and Pb-Si alkoxide solutions, respectively. PT alkoxide solutions were also prepared with excess Pb and Ti (7 and 10 mole %). Gels were prepared through controlled additions of water. Crystalline phase development of gel-derived powders with heat treatment (400-700 °C) was studied using x-ray diffraction (XRD) and differential thermal analysis (DTA). While PT powders without added Si crystallized directly into a perovskite phase, Si modified materials crystallized first into a pyrochlore phase and at a higher temperature transformed into perovskite. The pyrochlore lattice parameter and the temperature for the transformation to perovskite increased with Si content. In all cases, the crystal structure of the final perovskite phase was not affected by the Si addition. The effect of Si on phase development and mechanisms of transformation is discussed.
I. INTRODUCTION Lead titanate, PbTiO3 (PT), is a well-known ferroelectric ceramic with useful dielectric, pyroelectric, and piezoelectric properties.1'2 At room temperature, PbTiO3 has a tetragonal perovskite structure and, when combined with other perovskites, forms versatile crystalline solutions, such as Pb(Zr 1_xTiJ)O3 (PZT) 1 and Pb[(Mg1/3Nb2/3)1^Ti^]O3 (PMNT).3'4 These PT-based families of ferroelectric, paraelectric, and antiferroelectric materials have applications as capacitors,5 actuators,6 and sensors.2 Recent research has focused on the preparation and properties of thin layers or thin films of ferroelectric ceramics for a new generation of integrated components and devices, including FRAM (ferroelectric random access memory) elements,7 microactuators,8 pyroelectric detectors,9 and thin-layer capacitors.4'10 Because PT is a candidate material for these applications, some investigations have focused on the processing of PT thin films by a variety of methods, including sputtering,11 chemical vapor deposition,12 and sol-gel.13"15 The sol-gel route is well suited to the preparation of powders and thin films for electrical applications.10'15'16 This solution deposition method allows control over stoichiometry through molecular level mixing and is thus advantageous for the preparation of multicomponent oxides, such as PT. Other advantages of the sol-gel route include compositional variety, low-level doping capability, simplicity, and low cost.16 The procedure begins with the synthesis of a polymerizable solution, typically based on metal alkoxides in an alcohol solvent. The solution is diluted and partially hydrolyzed, and then deposited onto a substrate by a spinning or dipping operation. A polymeric gel network forms during deposition, 1712
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J. Mater. Res., Vol. 8, No. 7, Jul 1993
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and the resulting amorphous gel layer is heat-treated for organic removal and crystallization. Al
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