Structural studies of novel bismuth containing piezoelectric ceramics

Abstract

Perovskite-based materials are in the focus of research not only because of their excellent physical properties, but also because their relatively simple structure facilitates the understanding of structure-property relationships, which is crucial for developing novel materials with improved qualities. Recent research in the field of ferroelectric and piezoelectric materials is concerned with the development of eco-friendly lead-free materials. To achieve this goal, it is important to understand the fundamental correlation between the ‘Structure’ and the ‘Property’. In this work, the primary focus has been to elucidate the structural changes occurring as a function of doping in three different systems: (1) BiScO3-PbTiO3 (BS-PT), a recently developed system which has already attracted much interest because of its superior physical properties near the morphotropic phase boundary (MPB); (2) BiScO3-BaTiO3 (BS-BT), which can be considered as a lead-free analogue of the BS-PT family and lastly, (3) Na0.5Bi0.5TiO3-BaTiO3 (NBT-BT), which is a well-known lead-free material at the NBT-rich side of the phase diagram. Powder samples with a range of compositions for each system were prepared following the solid-state synthesis route and were investigated utilizing both neutron and x-ray powder diffraction and dielectric measurements. Detailed crystallographic information was obtained by Rietveld refinement against the neutron powder diffraction data. Structural phase transitions as a function of temperature were determined by nonambient x-ray powder diffraction and compared with the physical properties of the ceramics using high-temperature dielectric measurements. The significant outcomes are: 1. The best model to represent the so-called MPB of xBS-(1-x)PT system is found to be a mixture of a tetragonal and a monoclinic phases from the powder diffraction data. The structure beyond the MPB compositions is in better agreement for a single monoclinic model with the space group Cm than the accepted space group R3m. By contrast, single crystals with compositions around the MPB provide evidence for a model consisting of two primitive monoclinic cells. 2. The lead-free BS-BT system exhibits an extended phase boundary between tetragonal and pseudocubic phases, which can be modelled by a combination of tetragonal and rhombohedral phases. The incorporation of BS into BT also results in the suppression of the two low-temperature phase transitions of BT. 3. Samples with new compositions synthesized in the xNBT-(1-x)BT system demonstrate a rare enhancement in the tetragonality of the unit cell and an increase in the Curie temperature for compositions where x <= 0.40