Determinação da evolução estrutural em função da temperatura de cerâmicas ferroelétricas relaxoras do sistema PLZT

Abstract

The present work describes the analysis of the structural properties of lanthanummodified lead zirconate titanate (PLZT) ceramics as a function of temperature, in order to identify the crystalline phases present. The refinement of such samples, was performed by the Rietveld method, using the refinement program General Structure Analysis System (GSAS), by means of diffraction profiles at high resolution, obtained at the facilities of the National Synchrotron Light Laboratory, specifically, through the XPD beamline, subject to: E = 9.00352 keV, λ = 1.3775 Ǻ, step scan (2θ between 15° and 120°, with 0.02° step, 1s per step), with Ge analyzer (111). At first, by means of the single phase refinement protocol, the cubic lattice crystal symmetry was admitted, for all profiles; subsequently, the structural refinement used distinct crystal symmetries, all predicted for PLZT: the tetragonal crystalline system, the orthorhombic crystalline system, and the rhombohedral crystalline system. However, through the results obtained from this refinement, it was observed that, the orthorbombic and rhombohedral crystalline systems, did not present coherence in the refinement, although described in the phase diagram of PLZT. The results also suggested that the material presents tetragonal symmetry (P4mm) below TB and above TB, cubic symmetry (Pm-3m). The analysis of the agreement factors obtained in this stage of the study, however, showed that a dual phase refinement would be necessary to describe the PLZT system. Thus, considering the presence of two crystalline symmetries in the samples, refinements were performed, and the results were compared to the single phase refinements, the dielectric profile of the system, and the phase diagram of PLZT. It was observed that the PLZT 9/65/35 composition, as substantiated by previous studies and the literature, consists of a composition of tetragonal-cubic crystalline symmetries, evolving to a single cubic symmetry state by the influence of increasing temperature.