Estudo de propriedades estruturais, vibracionais, ópticas e magnéticas em amostras de Pb1-xCoxTiO3: transição de fase induzida por composição, temperatura e pressão

Abstract

This work presents a study of the synthesis and characterization of structural, vibrational, optical and magnetic properties of the compound Pb1-xCoxTiO3 (PCoT). The PCoT samples were synthesized using the Solid State Reaction method using a vibratory mill in the range of compositions at x ranging from pure PbTiO3 (PT, x = 0) to pure CoTiO3 (CoT, x = 1), (0 ≤ x ≤ 1), passing through the solid solution region, where the Co ions are diluted in the PT matrix (x < 0.1), advancing to the composite region, where the insertion of Co saturates the PT matrix and the CoT phase begins to form. These samples were initially characterized by Raman spectroscopy and x-ray diffraction (XRD) at ambient conditions (temperature and pressure) to determine the ideal conditions to synthesize each PCoT composition without spurious phase segregation, as well as to quantify the PT and CoT portions formed and analyze the effects of Pb-Co substitution on the structural and vibrational properties of these samples. Through Raman spectroscopy measurements as a function of temperature or pressure, it was possible to determine the influence of the composition on the PT (tetragonal to cubic) phase temperature (TC) or pressure (PC) transition, which presented a minimum of TC and PC at 720 K and 9 GPa, respectively, in the range of 0.3 ≤ x ≤ 0.5. Through XRD measurements as a function of pressure, it was possible to confirm the effect of the composition on the PC and to estimate the compressibility values and the bulk modulus (B0) for different PCoT compositions, so that B0, initially 103 GPa (x = 0), reduced to 84 GPa at x = 0.5, confirming an improve of compressibility induced by the Pb-Co substitution. Light absorption measurements in the UV-Vis region show that Pb-Co substitution reduces the optical gap of samples from 3.1 eV in the PT (x = 0) to 2.5 eV in the sample x = 0.1, while the CoT sample shows energy gap of 2.6 eV. The analysis of the magnetic properties through measurements of magnetic susceptibility as a function of temperature (χ × T) and measurements of magnetization as a function of the magnetic field (M × H), suggest that for low concentrations of Co (x < 0.1) there are a typical behavior of paramagnetic materials, while above x = 0.1 it is possible to observe the presence and establishment of the CoT phase formation due to the confirmation of the TN (38 K), typical of materials with antiferromagnetic order. Furthermore, at T = 5 K samples with composition x < 0.4 exhibit coercive field (HC) and remaining magnetization through M × H measurements, with 1500 Oe of HC, suggesting the presence of weak ferromagnetism at this temperature due to the possibility of interaction between Co ions diluted in the PT matrix.