Materials Chemistry and Physics 112 (2008) 886–891 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys A homovalent doping in PMN ceramics by using lithium and scandium cations A.J. Paula a,∗ , A.A. Cavalheiro b , J.C. Bruno b , M.A. Zaghete a , Elson Longo a , J.A. Varela a a Liec, Instituto de Química, UNESP, Rua Prof. Francisco Degni, s/n, Zip Code 14.801-970, P.O. Box 355, Araraquara, SP, Brazil b Depto. de Química, Instituto de Biociências, UNESP, Distrito de Rubião Junior, s/n, Zip Code 18.618-000, P.O. Box 510, Botucatu, SP, Brazil article info Article history: Received 23 April 2008 Received in revised form 24 June 2008 Accepted 25 June 2008 Keywords: Pb(Mg 1/3 Nb 2/3 )O3 Ceramics Powder diffraction Ferroelectrics Rietveld analysis abstract The Mg 2+ homovalent substitution by Li + /Sc 3+ cations at the B-site sub-lattice of the lead magnesium niobate (PMN) perovskite structure was studied in this work. Through structural and electrical analyses, it was observed that the dopant pair effectively substituted the Mg cation, although an addition limit close to 2.5mol% of dopants was observed. Up to this concentration level, the dopant pair does not affect the perovskite phase stability, and thus, pyrochlore free ceramics with a T m value of -6 ◦ C and 19,000 of electric permittivity were obtained. As a consequence of the substitution, the 1:1 chemical ordering at the B-site was changed, leading to a less diffuse ferroelectric–paraelectric phase transition even despite the increase of the relaxor character. However, a 5.0mol% addition of Li/Sc provoked the precipitation of a small quantity of pyrochlore phase that caused a deterioration of the ceramic electrical properties. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The dielectric properties of the relaxor ferroelectrics (RE) have attracted attention of many researchers around the world. Instead of the macro-domains present in normal ferroelectrics, the complex-oxide perovskite structure A(B’B”)O 3 has essentially a random lattice order–disorder effect that leads to the forma- tion of nano-domains, which are responsible for the occurrence of the relaxor characteristics [1–3]. These materials possess a diffuse phase transition and a singular relationship between permittivity and temperature as a function of the applied frequency. As a con- sequence, the permittivity values above the Curie temperature (T c ) do not decrease abruptly. Among the RE-materials, the lead mag- nesium niobate (Pb(Mg 1/3 Nb 2/3 )O 3 ) has been intensively studied because of its applications as multilayer capacitors, actuators and electromechanical devices [4–7], which are granted due to its great volumetric efficiency (K m > 20,000), high electrostrictive properties and low sintering temperatures [8]. Despite the maximum permittivity (K m ) of PMN occurring at negative temperatures (-10 < T m <0 ◦ C), this property can be sig- nificantly affected when modifiers are inserted into the perovskite structure. For instance, the substitution at the A site (Pb 2+ ) changes the electrical properties and sintering conditions. On the other hand, studies related on substitutions at the B sites (Nb 5+ , Mg 2+ ) have confirmed the variation of T m and K m values without causing ∗ Corresponding author. Tel.: +55 16 3301 6600/6865; fax: +55 16 3322 7932. E-mail address: amauri jp@yahoo.com.br (A.J. Paula). significant losses in the electrical properties [9,10]. At tempera- tures above T m , the crystal tends to assume a uniform primitive cubic (Pm ¯ 3m) structure, and when T m values is raised or changed by doping, the crystal structure becomes pseudo-cubic, increasing the spontaneous polarization at room temperature [11]. In this context, the substitution of the Mg 2+ in the Pb(Mg 1/3 Nb 2/3 )O 3 perovskite structure by a pair of dopants (Li + and Sc 3+ ) is now proposed. When used in a 1:1 ratio, these cations enable the conservation of the charge neutrality, avoiding the creation of extrinsic vacancies. Besides, a recent study using a Li additive showed improvements in the dielectric properties of 0.9PMN–0.1PT [12]. However, there are no studies available con- cerning the effects of using the Sc atom as a dopant [13,14]. In this way, features as perovskite phase percentage, powder morphology, density and microstructure were correlated in order to explain the PMN electrical properties of the Li/Sc modified PMN ceramics. 2. Experimental Niobium oxide (Nb2O5) (99.9%, Alfa Aeser), basic magnesium carbon- ate ((MgCO3)4Mg(OH)2·5H2O) (>99%, Cinética Química), lithium carbonate (LiCO3) (>99%, Vetec), scandium oxide (Sc2O3) (99.9%, Alfa Aeser) and lead oxide (PbO) (>99%, Synth) were used as starting materials to prepare (Pb(Mg (1-x) ) 1/3 Nb 2/3 )O3 - x(Li 1/2 Sc 1/2 ) powders, where x = 0.0, 0.025 and 0.050. The samples will be hereafter designated as PMN-P, PMN-25LS and PMN-50LS, respec- tively. The synthesis was conducted following the columbite route [15], where the precursors of Mg, Nb and Li/Sc dopants were mixed and ball milled for 8 h in order to obtain powders with high surface area. After drying, the resulting powder was sieved using a 200-mesh sieve and calcined at 1200 ◦ C for 4 h to obtain the columbite phase. After preparing the MN powder, the columbite precursor was ball milled for 16 h in isopropyl alcohol, dried and then reacted with PbO (1 wt.% in excess) via a solid-state reaction at 850 ◦ C for 2h. To prepare the PMN ceramics, the powders 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.06.061