Piezoelectric and optical properties of Sr-doped PT–PZ–Pb(Mg_1/3Nb_2/3)O₃ ceramics

Abstract

Ceramics in the ternary system (Pb,Sr)TiO₃–PbZrO₃–Pb(Mg_1/3Nb_2/3)O₃, in the ratio 85.976/5.817/8.207 mol%, was prepared by mixed oxide route. With about 13.98 at.% Sr⁺², as isovalent substitution in A-position of the lead titanate, high-density samples were obtained for the sintering temperatures of about 1220 and 1230 °C. Tetragonal phases were identified both by XRD method and selected area electron diffraction pattern (SAED). The lattice parameter ratio, c/a, of about 1.017 indicated a decrease of tetragonal distortion. The typical TEM micrographs showed domains of about 50 nm wide. The reflection spectra were recorded to prove the presence of the transition Ti⁺⁴ to Ti⁺³. The principal mechanical, electrical and electromechanical properties were discussed as a function of the sintering temperature. The variation of the relative constant with the temperature and frequency evidenced a relaxation-like behavior of this ternary system. By combining the ferroelectric and relaxor end member we tried to improve the piezoelectric properties of a ceramic with a high content of lead titanate, and to achieve a material with both piezoelectric and relaxor-like properties.

Introduction

The unmodified lead titanate is an ABO₃-type compound with a tetragonal structure at room temperature. The distortion of PbTiO₃ unit cell is about 1.063, much greater than in BaTiO₃. The lattice parameters vary with the temperature and hydrostatic pressure. Pure PbTiO₃ (PT) cannot be used as a material for piezoelectric transducers, but with various substitutions (isovalent-, compensating valence- and A-vacancy substitutions), new materials with piezoelectric properties were obtained. Sr⁺², as isovalent substitution for Pb⁺² in PT, decreases the Curie point of the ceramic and reduces the tetragonal c/a ratio. A Curie point of about 180 °C is reported for a PT composition with 50 at.% Sr⁺². Is important to remark that for compounds with high concentration in Pb⁺² and small content in Sr⁺², no piezoelectric properties have been reported.

Strong piezoelectric effects are observed for PbTiO₃ modified with other compounds, near the morphotropic phase boundary, where co-exist both rhombohedric and tetragonal phases. The composition of the morphotropic phase is specific for each compound or substituent added to lead titanate. The molar content of PT varies from 5–6 mol% to 35 mol% and 47–48 mol% for the PT–Pb(Fe_0.5Nb_0.5)O₃, PT–Pb(Mg_1/3Nb_2/3)O₃ and PT–PbZrO₃, respectively. For these compositions large planar coupling factors of nearly 0.6 and dielectric constant over 3000 have been reported. Many studies are made on the piezoelectric properties of binary and ternary systems near the morphotropic phase boundary, with the PT maximum content of about 48–50 mol%.1, 2 None studies were reported concerning ternary systems with a PT content more than 80 mol%.

The aim of this contribution is to study a ternary system composed from lead titanate, lead zirconate and Pb(Mg_1/3Nb_2/3)O₃, with a high content of PT, more than 85 mol%. We have chosen a mixture of about 85.976 mol% Sr⁺²-modified PT and 5.817 mol% PbZrO₃ (PZ), far from the morphotropic phase boundary in the PT–PZ phase diagram, with an unique ferroelectric tetragonal phase. In the lead titanate, 13.98 at.% Pb⁺² were isovalent substituted with Sr⁺². At the above mixture we added 8.207 mol% Pb(Mg_1/3Nb_2/3)O₃, a well-known relaxor compound, in order to improve the piezoelectric properties of the system. The principal mechanical, electrical and electromechanical properties were determined as a function of the sintering temperature. Using transmission electron microscopy (TEM) and X-ray diffraction method we have investigated the microstructure of the sample sintered at 1220 °C. The ternary system contains a high concentration in Ti⁺⁴ together with, as additives, both isovalent ions (Sr⁺², Mg⁺²) for Pb⁺² substitution and donor ions (Nb⁺⁵) for Ti⁺⁴ substitution. For this reasons it is expected that Ti⁺⁴ can reduce to Ti⁺³ in order to compensate Nb⁺⁵. For evidencing this transition, reflection spectra were recorded. The presence of the niobate member of the ternary system will induce a relaxation-like behavior, so the dependence of the relative dielectric constant with the temperature and frequency was studied.