Kinetics of formation of the pyrochlore and perovskite phases in sol-gel derived lead zirconate titanate powder
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Kinetics of formation of the pyrochlore and perovskite phases in sol-gel derived lead zirconate titanate powder V. S. Tiwari, Arun Kumar, and V. K. Wadhawan Crystal Growth Laboratory, Centre for Advanced Technology, Indore-452013, India
Dhananjai Pandey Department of Physics, Banaras Hindu University, Varanasi-221005, India (Received 14 February 1997; accepted 29 December 1997)
Lead zirconate titanate (PZT) powder is prepared by the sol-gel method. The formation of pyrochlore and perovskite phases is investigated by high temperature x-ray diffraction (XRD) and thermal analysis techniques. The pyrochlore phase first appears in x-ray amorphous form, and then gets converted to crystalline state on annealing in air. We show that vacuum annealing of the pyrolyzed amorphous PZT gel suppresses the formation of the crystalline pyrochlore phase. This, in turn, enhances the kinetics of conversion of pyrochlore to perovskite, such that a pyrochlore-free perovskite phase can be obtained by annealing at about 500 ±C. On the other hand, if annealing is carried out in air, a crystalline pyrochlore phase is formed, which requires annealing temperatures higher than 600 ±C for transformation to the perovskite phase. These observations are explained tentatively in terms of the oxygen stoichiometry of the two phases.
I. INTRODUCTION
Lead zirconate titanate, Pb(Zrx Ti 12x )O3 (PZT), is a well known ferroelectric material for applications in piezoelectric, electrooptic, and pyroelectric devices in bulk ceramic form.1,2 Recent years have witnessed immense interest in thin films of PZT for a variety of device applications, such as micromechanical devices,3–5 infrared detectors,6 and ferroelectric memories.6–8 The possibility of integrating the thin film deposition technique with the existing semiconductor microelectronics processing has provided additional impetus to the activities on PZT thin films.6,9,10 The sol-gel route has emerged11–16 as the most versatile method for preparing chemically homogeneous PZT powders as well as thin film coatings. However, one of the major limitations of the sol-gel route is that it does not yield the desired perovskite (Pe) phase (needed for the device applications) directly after the pyrolysis of the gel, either in bulk or in thin film form. The formation of the Pe phase is, instead, mediated by an undesirable pyrochlore (Py) phase which is formed17,18 first after pyrolysis. Several different factors, such as pyrolysis and subsequent heat-treatment temperatures,16 annealing atmosphere,19–21 PbO content,13,22 and the nature and temperature of the substrate,19–21,23 seem to influence the transformation of the pyrochlore phase into the perovskite phase. However, literature abounds with contradictory reports about the exact role of these factors, especially the oxygen atmosphere.19–21 A proper understanding of the kinetics of the Py to Pe transformation 2170
http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 8, Aug 1998
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