STUDY OF 0.9PMN-0.1PT DIELECTRIC BEHAVIOR IN RELATION TO THE NANOSTRUCTURE

(1 - x)PbMg1/3Nb2/3O3-xPbTiO3 ceramics with a = 0, 0.1 were prepared w ith a 12 mol% MgO excess to obtain dense and perovskite phase material s after sintering. The dielectric characterization has revealed that a local polarization appears at a Td temperature largely above the temp erature of the maximum of permittivity (T-d, respectively -13 degrees C and +36 degrees C for z = 0 and 0.1). This phenomena is consistent w ith the nucleation of polar clusters. Moreover, a dielectric relaxatio n is observed for 0.9PMN-0.1PT-0.12MgO, in a large frequency range (10 0 Hz - 1 GHz), which corresponds to a multi-Debye process with broaden ing of the relaxation time distribution as the temperature decreases. This suggests a nucleation and growth mechanism of polar clusters with decreasing temperature, which can result from the successive transiti ons of different compositions. This hypothesis was confirmed by the id entification of large chemical heterogeneities on a nanometric scale b y TEM using two spectroscopy techniques (EDXS and EELS); because of th e association of low and high atomic number elements in the materials, different types of equipment and also the simulation of the patterns with standards. In fact, these quantitative analyses have revealed lar ge fluctuations of the local composition around the nominal one: lead and magnesium deficient areas enriched in niobium coexist with nanodom ains largely enriched in lead and slightly in magnesium, which the siz e depends on the titanium content. The origin of these heterogeneities in correlation wish the reactions sequences during calcination and si ntering is discussed: in particular the addition of titanium contribut es, by stabilizing the perovskite phase, to limit the diffusion of lea d oxide, which consequently increases the homogeneity of the ceramics. Due to such heterogeneities, the material remains mainly paraelectric up to very low temperatures. This effect can be balanced by the appli cation of a high electric field which induces the growth of the polar clusters by displacement of their interface with the paraelectric matr ix and orientation of their polarization in the direction of the elect ric field which can lead to a macroscopic ferroelectric transition in specific conditions of temperature and electric field intensity. These different mechanisms relax in a frequency range which depends on the temperature and on the amplitude of the electric field.