Ferroelectric Behavior of Pb(Mg 1/3 Nb 2/3 )O 3 (PMN) Obtained by the Sol-Gel Method Purificacio ´n Escribano,* ,² He ´ctor Beltra ´n, ² Eloisa Cordoncillo, ² Germa ` Garcı ´a-Belmonte, ‡ Luisa Ruiz, § Jose ´M a Gonza ´ lez-Calbet, § and Anthony R. West | Departamento de Quı ´mica Inorga ´ nica y Orga ´ nica and Departamento de Ciencias Experimentales, Universitat Jaume I, 12080 Castello ´ n, Spain, Departamento de Quı ´mica Inorga ´ nica, Facultad de Quı ´micas, Universidad Complutense, 28040 Madrid, Spain, and Department of Engineering Materials, University of Sheffield, Mappin Street, Sheffield S1 3JD, United Kingdom Received June 29, 2000. Revised Manuscript Received October 25, 2000 The ferroelectric behavior of lead magnesium niobate perovskite, obtained by sol-gel methods, has been investigated in three samples with compositions based on Pb(Mg 1/3 Nb 2/3 )O 3 (PMN) to which an excess of PbO and MgO has been added. Electron probe microanaly- sis (EPMA) showed a single phase of stoichiometry PbMg 0.33 Nb 0.68 O 3.02 in sample 2, PbMg 0.36 Nb 0.69 O 3.09 in sample 3, and PbMg 0.32 Nb 0.67 O 3 together with PbO in sample 4. Permittivity measurements reach values higher than 22 000 for sample 2 and 11 000 for samples 3 and 4. A microstructural characterization of the first two samples, by means of high-resolution electron microscopy, shows the presence of small domains of double periodicity inserted in a matrix with a single cubic perovskite structure. The concentration of double domains is higher in the first sample. The third sample shows a mixture phase due to the presence of PbO. The relationship between microstructure and electrical behavior shows that the permittivity values are related not only to the existence of a single PMN phase but also to Pb:(Mg + Nb) ratios close to 1 and Mg:Nb ratios close to 0.5. Introduction Lead magnesium niobate, Pb(Mg 1/3 Nb 2/3 )O 3 (PMN), is an important relaxor ferroelectric material that exhibits a high dielectric constant and a high electrostrictive strain coefficient. X-ray diffraction studies are in agree- ment with an average cubic ABO 3 perovskite [space group (S.G.) Pm3m], at room temperature, which would involve Mg and Nb atoms randomly distributed on the B sites. 1,2 However, selected area electron diffraction (SAED) and high-resolution transmission electron mi- croscopy (HRTEM) studies 3,4 reveal the existence of microdomains, showing a 2-fold perovskite superstruc- ture, inserted in a cubic matrix. Such microdomains could proceed from Mg and Nb ordering, within a rich Nb matrix. The dielectric properties of complex lead-based per- ovskite materials, with the general formula Pb(B′ x B′′ 1-x )O 3 , depend on B-sites ordering. If these sites are randomly occupied, a normal ferroelectric behavior, with sharply defined Curie transition temperatures, is observed. However, a relaxor ferroelectric is obtained if cations are ordered. Dielectric constants unusually high are observed over a wide temperature range for these relaxor materials. The temperature at which the dielectric constant is maximal shifts to higher values and decreases when frequency increases. In relaxor ferroelectrics, structural disorder in B-sites has been proposed to disrupt the translational crystal symmetry, giving an apparent anisotropic component for some measured electrical properties. 5-7 Several techniques have been used to study the local B-site distortions providing atomic-level explanations for the high dielectric properties of PMN: (a) long-range B-site distortions may occur causing deviations from cubic crystal symmetry at low temperature and (b) short-range B-site changes involving off-center shifts or movement in the B-sites. These structural changes seem to be responsible for local positive and negative charge separation in these materials that creates a bulk spon- taneous polarization. In addition, HRTEM, 8 EXAFS, 9 and single-crystal X-ray diffraction 10 studies have shown the presence of domains due to a partially ordered * To whom correspondence should be addressed. ² Departamento de Quı ´mica Inorga ´nica y Orga ´ nica, Universitat Jaume I. ‡ Departamento de Ciencias Experimentales, Universitat Jaume I. § Universidad Complutense. | University of Sheffield. (1) Bonneau, P.; Garnier, P.; Husson, E.; Morell, A. Mater. Res. Bull. 1989, 24, 201. (2) Smolenskii, G. A.; Siny, I. G.; Pisarev, R. V.; Kuzminov, E. G. Ferroelectrics 1976, 12, 135. (3) Husson, E.; Chubb, M.; Morell, A. Mater. Res. Bull. 1988, 23, 357. (4) Chen, J.; Chan, H. M.; Harmer, M. H. J. Am. Ceram. Soc. 1989, 72, 593. (5) Glazer, A. M.; Mabud, S. A. Acta Crystallogr. 1978, B34, 1060. (6) Gupta, S. M.; Kulkarni, A. R. Mater. Chem. Phys. 1994, 39, 98. (7) Zhukov, S. G.; Chernyshev, V. V.; Aslanov, L. A.; Vakhrushev, S. B.; Schenk, H. J. Appl. Crystallogr. 1995, 28, 385. (8) Bursill, L. A.; Qian, H.; Peng, J.; Fan, X. Physica B 1995, 216, 1. (9) Ye, Z. G. Ferroelectrics 1996, 184, 193. (10) Prouzet, E.; Husson, E.; de Mathan, N.; Morell, A. J. Phys. Condens. Matter 1993, 5, 4889. 415 Chem. Mater. 2001, 13, 415-419 10.1021/cm0011108 CCC: $20.00 © 2001 American Chemical Society Published on Web 01/10/2001