Materials Chemistry and Physics 84 (2004) 107–111 Effect of PbO excess on the formation of lead magnesium niobate perovskite by the columbite method A.A. Cavalheiro ∗ , S.M. Barrionuevo, J.C. Bruno, M.A. Zaghete, M. Cilense, J.A. Varela LIEC—Instituto de Qu´ ımica—UNESP, Zip Code 14.801-970, P.O. Box 355, Araraquara, SP, Brazil Received 1 April 2003; received in revised form 20 October 2003; accepted 7 November 2003 Abstract The effect of lead excess on the pyrochlore-type formation in Pb(Mg 1/3 Nb 2/3 )O 3 (PMN) powders has been investigated. The polymeric precursor method was used in the synthesis of the columbite in association to the partial oxalate method to synthesize the PMN powder samples. Structure refinement of the columbite precursor and PMN powders was carried out using the Rietveld method. The quantitative phase analysis showed that the amount of perovskite phase is not affected by PbO excess, but a great excess drives the pyrochlore-type formation so that 3 wt.% of PbO causes the predominance of Mg-containing pyrochlore phase. Using the refined data obtained from the Rietveld refinement, the compositional fluctuation in the perovskite phase was calculated from Nb/Mg ratio values and Pb occupation factor. Mg inclusion occurs concomitant with Pb one into PMN perovskite phase and this effect is directed by PbO excess during powder synthesis. © 2003 Elsevier B.V. All rights reserved. Keywords: Ceramics; Pb(Mg 1/3 Nb 2/3 )O 3 ; Chemical synthesis; Phase transitions; Rietveld method 1. Introduction The relaxor ceramic Pb(Mg 1/3 Nb 2/3 )O 3 (PMN) exhibits high dielectric constant and electrostrictive characteristics, that make this material so useful for present and future elec- tronic and microelectromechanical applications [1–4]. The central problem in manufacturing PMN-based single-phase ceramics is the strong inclination of this material to form detrimental secondary pyrochlore (Py) phases. The py- rochlore phases in PMN are known as compounds with a low content of magnesium, whose structure is more ener- getically stable than the one of the perovskite (Pe) phase. The presence of these secondary phase harms the dielec- tric and electrostrictive properties of PMN-based ceramics [5,6]. A synthesis route, named columbite method [7], was pro- posed to reduce the pyrochlore phase to a minimum level in PMN powders. In this methodology, MgO and Nb 2 O 5 are initially made to react to obtain the MgNb 2 O 6 precursor, and in sequence, it will react with a stoichiometric amount of PbO to obtain PMN powders with a great amount of the perovskite phase. ∗ Corresponding author. Tel.: +55-16-201-6712; fax: +55-16-222-7932. E-mail address: albecava@bol.com.br (A.A. Cavalheiro). A variation of the columbite method [8–11] was exten- sively studied and includes polymeric intermediate phases to synthesize the columbite precursor. The main advantages of this method are the short time and low temperature used in the heat treatment and better powder homogeneity due to a uniform cation distribution in the polyester. This homo- geneity of the polymeric precursor before thermal decom- position permits the direct crystallization of the MgNb 2 O 6 orthorhombic structure after burning. This method will be now referred as columbite method via polymeric precursor. The fundamental step to obtain PMN powders with a great amount of perovskite phase is the reaction between PbO and the columbite precursor. PbO volatilization is the main occurrence factor when the pyrochlore amount increases in PMN powders, and in this way, many artifices can be used to avoid the formation of the pyrochlore phase [12–14]. PbO excess is one of them, because it aims to compensate PbO volatilization during heat treatment [15–17]. The role of PbO excess on the general characteristics of PMN powders is not clear and some authors reported only indirect effects on PMN bulk ceramics, as variations of the dielectric properties and microstructure [14–16]. However, an improvement or impoverishment of the final properties depends much more directly on the processing, firing con- ditions and apparatus used in sintering stage, making the 0254-0584/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2003.11.013