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MATERIALS RESEARCH

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Phase analysis of seeded and doped PbMg1//3 Nb2//3 O3 prepared by organic solution of citrates J. C. Carvalho, C. O. Paiva-Santos, M. A. Zaghete, C. F. Oliveira, and J. A. Varela I.Q.-UNESP. 14800-900, Araraquara, S. P., C.P. 355, Brazil

E. Longo DQ-UFSCar. 13560-905, S. Carlos, S.P., C.P. 676, Brazil (Received 15 November 1994; accepted 18 March 1996)

PbMg1/3 Nb2/3 O3 (PMN) prepared by organic solution of citrates was analyzed by the Rietveld method to determine the influence of seeds and dopants on the perovskite and pyrochlore phase formation. It was observed that pyrochlore phase formation increases with an increase in calcination time when no additives are included during the preparation. It was also observed that a greater amount of perovskite phase appeared in doped or seeded samples. The fraction of perovskite phase increased from 88 mol % in pure sample to ,95 mol % in doped and seeded samples calcined at 800 ±C for 1 h. It is clear that the addition of dopants or seeds during PMN preparation can enhance the formation of perovskite phase.

I. INTRODUCTION

PbMg1/3 Nb2/3 O3 (PMN) has been studied since 1950.1 It has perovskite (Pe) type structure ABO3 2 at room temperature and characteristics that make it useful as dielectric in multilayer ceramic capacitors.2–5 These characteristics are low sintering temperature (,1000 ±C), high dielectric constant (8000–15,000), and a high electrostriction coefficient6 (0.1%). These properties depend on the precursors’ purity, the processing method, as well as the reaction time and temperature. Any change in these parameters can induce the formation of pyrochlore phase (Py). This phase has a low dielectric constant (130–200)7 that causes the overall dielectric constant of PMN to decrease. The PMN was first prepared by mechanical mixture of oxide precursors3,5 that promote the formation of pyrochlore phase. Aiming to get PMN free of pyrochlore phase, several PMN preparation methods have been performed. Swartz and Shrout5 prepared perovskite PMN at 800 ±C by using columbite precursor which was then prepared at 1000 ±C; even so, 2% of pyrochlore phase was present. Ravindranathan et al.2 prepared PMN by the sol-gel method where Pe phase could be formed at 775 ±C. In this work, they were able to prepare pure perovskite type PMN by seeding the material with 1 wt. % of Pe PMN, also taking advantage of lowering the crystallization temperature by 75 ±C. Liou and Wu8 could enhance the dielectric properties of PMN by adding PbTiO3 and excess of MgO and PbO. Among all the methods described in the literature, chemical routes have shown better performance to decrease the amount of pyrochlore phase. In this work PMN has been prepared by organic solution of citrates9 using the concept of controlled J. Mater. Res., Vol. 11, No. 7, Jul 1996

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nucleation10 through the introduction of BaTiO3 seed particles or barium/titanium doping solution. II. EXPERIMENTAL

Precursor reagents used were nio