Pyrochlore-to-perovskite transformation during rapid heating of sol-gel (Pb,La)TiO 3 thin films
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Pyrochlore-to-perovskite transformation during rapid heating of sol-gel (Pb,La)TiO3 thin films Miguel Alguero´, M. Lourdes Calzada, and Lorena Pardo Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, Cantoblanco 28049, Madrid, Spain (Received 16 July 1998; accepted 30 August 1999)
Phases appearing in lanthanum-modified lead titanate thin films prepared by a diol-based sol-gel method and crystallized by rapid heating were studied. The results clearly indicate that a phase transformation from a pyrochlore structure to the perovskite phase occurs in Pb-deficient films during the thermal treatment, which involves a heating rate higher than 500 °C min−1. The rate of this transformation is a function of the lead content of the films, decreasing as lead volatilizes. Temperatures higher than 650 °C or soak times longer than 2 h make possible the complete pyrochlore-to-perovskite transformation without any lead excess in the films. I. INTRODUCTION
In the study of ferroelectric thin films of lanthanummodified lead titanate, (Pb,La)TiO3, hereinafter referred to as PTL, we have reported the development of a second phase coexisting with the desired perovskite-type structure.1,2 It was shown2 that this second phase is a Pbdeficient phase, and that it does not appear when an excess of PbO is added to the precursor solution. Pbdeficient phases have been also found in other lead-based perovskite films, as in Sm-modified lead titanate3 and in lead zirconate titanate (PZT) films.4,5 Their origin is being investigated intensively. It is accepted that the second phase is a stabilized intermediate phase6–8 initially developed as an amorphous phase, and later transformed into perovskite. PbO volatilization causes the formation of Pb-deficient areas in which the second-phase-toperovskite transformation is inhibited. Moreover, high Pb deficiencies may lead to its transformation into higher Pb-deficient phases, as modifications of the monoclinic structure PbTi3O7.5,9 An important issue in ferroelectric thin films regarding their use as active elements in memories,10,11 infrared sensors,12 and microelectromechanical systems13 is their integration with Si circuits, which requires a low “thermal budget” involved to prevent the circuit damage. One approach is sol-gel processing using rapid thermal annealing, in which heating rates of several tenths of °C s−1 are used.14 There is some controversy on the effect of raising the heating rate in such a manner in the mechanisms of perovskite formation. It has been proposed that rapid thermal annealing allows the direct nucleation of the perovskite from amorphous layers deposited by multi-ion-beam-reactive sputtering.15 PZT films obtained by this method are free of second phase even when the temperature of crystallization and the soaking time 4302
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J. Mater. Res., Vol. 14, No. 11, Nov 1999 Downloaded: 19 Jan 2015
are as low as 450 °C and 1 s, respectively. An analogous mechanism ha
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