Local Structure of NBT-Based Piezoelectric Materials

This thesis explores local structure variation in (1-x)(Na0.5, Bi0.5)TiO3-xPbTiO3 (NBT-PT) and (Kx, Na1-x)0.5Bi0.5TiO3 (KNBT) around the morphotropic phase boundary (MPB). Local structure alignment or ordering in NBT-PT was achieved through the addition of PbTiO3 (PT), whilst in KNBT local ordering was induced by an applied electric field. Significant emphasis is placed on local structure analysis methods (up to 50 Å length scale) via pair distribution function (PDF) analysis.
In situ temperature X-ray diffraction was used to characterise the average structure of NBT-PT. A transition from a rhombohedral structure for x = 0.08 to tetragonal for x = 0.18 was observed (MPB x = 0.13). The ferroelectric-paraelectric transition temperature was corroborated by permittivity measurements which also showed a transition from relaxor to ferroelectric behaviour with increasing x. Whole profile PDF refinement revealed the presence of a monoclinic phase for x = 0.14 acting as a lower symmetry bridge between rhombohedral and tetragonal phases. Range dependent PDF analysis was used to measure the coherence length of nanoscaled regions which decreased in size from 40 to 20 Å with increasing x. These regions persisted at temperatures above the paraelectric transition, though reduced in size across all compositions. The measurements illustrate the order inducing properties of PbTiO3, which suppresses nanoregions and promotes long-range ferroelectric order.
PDF analysis of unpoled KNBT at unit-cell length scale distances was used to measure the local Bi off-centre displacement direction. For x = 0.10, a rhombohedral distortion was observed. This transitioned to a monoclinic distortion for x = 0.15, further evolving into a complex mixture of various monoclinic distortions for x = 0.20 (MPB). A tetragonal distortion was observed beyond the MPB (x = 0.30). The improved piezoelectric properties at the MPB are attributed to the greater availability of displacement directions. Under an applied electric field, the suppression in diffuse scattering and sharpening of PDF peaks indicating field induced ordering. Changes in the peak area ratios corresponding to Bi-Ti distances indicate reorientation behaviour along the applied field vector. Local strain analysis was achieved by measuring the PDF peak shift. The onset of linear strain corresponding to piezoelectric response occurred at an electric field (E) ≈ 1000 - 1250 V/mm for x = 0.20 and at E ≈ 2000 – 3000 V/mm for x = 0.15 and 0.18. Non-zero strain below the threshold field indicates the presence of localised strain assumed to be incipient to the macroscopic strain.