Diffuse scattering study of short-range order in Lead Zinc Niobate

Abstract

Lead zinc niobate (PZN) is an important ferroelectric and piezoelectric material, being one of the most commonly used materials for this purpose. The main aim of the project was to investigate the nanostructure of PZN using various scattering techniques; to gain a better understanding of the short-range order of the nanostructure and how it evolves with temperature. The diffuse scattering from lead zinc niobate (PZN) was collected and analysed from both powder and single crystal samples at a range of temperatures. The data collection and processing techniques were developed in order to allow collection of high quality single crystal diffuse scattering (SCDS) from the powder diffractometer Wombat at the Australian Nuclear Science and Technology Organisation (ANSTO). The evolution of the diffuse scattering with temperature was analysed. Results presented in this work show the evolution of the diffuse rods of scattering (a result of the planar nanodomain structure), the scattering as a result of size-effect relaxation and the short-range chemical ordering of the B-site atoms, Zn and Nb. The size-effect was observed to undergo a rapid change in intensity at around the Curie temperature, although all the other feature evolve slowly across the transition. This implies that the size-effect relaxation could be related to the ferroelectric properties. The SCDS was modelled using Monte Carlo (MC) simulations as a method to induce short-range order into the system. The B-site ordering used an Ising energy, while to induce the planar nanodomain formation an artificial MC energy is required. The development of the Global Instability Index (GII), which is the normalised sum of the square of the difference between expected and calculated bond valence parameters, as an MC energy term was done in order to induce the short-range order of the non-Pb atoms in the model in a chemically realistic manner. The final study used Total Scattering data which were collected using both X-rays and neutrons. The PDF data analysis of this data was performed using both a full-profile fitting approach and the reverse Monte Carlo (RMC) refinement of a larger model. The qualitative analysis of the PDF data showed that a lead displacement direction of <110> is closest to what is observed in the data. There were also observed to be different separation distances between the two different B-site atoms and the neighbouring O atoms. This is information that can not be as easily determined from the single crystal data. The evolution of the PDF data with temperature was also analysed. The RMC refinement produced very precise fits to the data and provided a small amount of insight to the short-range order. The comparison of the PDF and SCDS techniques gave insight to the advantages and disadvantages of the different techniques for analysis of diffuse scattering, and showed that while the PDF data is sensitive to the Pb atom displacements, it is not sensitive to the formation of the planar nanodomains. Show more