TEM Investigations on Bismuth-Based Perovskite Oxides

Abstract

<p>The research presented in this thesis aims to provide fundamental understanding about the structure-property relationships in Bi-based perovskite oxides from the nanoscale to the atomic level so that the properties of these materials can eventually be tuned. This study focused on the investigations of structure and ordering phenomena in several Bi-based compounds, namely a combination of BiFe03 (BFO) and BiCr03 (BCO) as well as a layered perovskite Bi4_xLax Ti30 12 (x = 0.75) (BLT), using a variety of transmission electron microscopy (TEM) techniques. The performance of the new generation of spherical aberration (Cs)-corrected TEMs with sub-A spatial resolution was also demonstrated.</p> <p>The investigations were performed using both conventional TEMs as well as the entirely new generation TEMs (based on aberration correction) as the major characterization instruments. Aberration-corrected TEMs were specifically utilized for analysis at the atomic level, the observation of the structure of defects and probing of valence changes. The importance and advantages of using a Cs-corrector in improving spatial resolution for observations of atomic structure were reviewed in detail.</p> <p>First, the Bi-based perovskite oxide films of BbFeCr06 (BFCO 1/1), consisting of BFO and BCO with a 1:1 ratio and epitaxially grown by the pulsed laser deposition (PLD) technique, were investigated in order to understand the remarkable increase in saturation magnetization. The selected area electron diffraction (SAED) and convergent beam electron diffraction ( CBED) experiments revealed various weak reflections, which were explained through systematic elimination of several possible factors. The extra weak reflections in the CBED pattern of the sample oriented with the zone axis <110> arose due to B-site cationic ordering and anti-phase tilting of B06 octahedra.</p> <p>Other weak reflections in the CBED pattern acquired at an orientation parallel to the <100> zone axis correspond to the reflections of a secondary phase, Bi20 3 , which was formed during the deposition of the film. Electron energy loss spectroscopy (EELS) experiments revealed that both B-site cations, i.e., Fe and Cr, possess the same valence of 3+.</p> <p>Second, the oxide films produced by the co-deposition of BFO and BCO in a 1:3 ratio, BbFe0.5Crl.506 (BFCO 1/3), were investigated to study the predicted B-site cationic ordering. Through high angle annular dark field (HAADF) imaging, our results revealed a structure that is different from the double perovskite structure previously expected. The two-dimensional XRD experimental results gave a 3D reciprocal space map, which showed patterns consistent with the SAED patterns and revealed the presence of twinning and variation of in-plane orientations in the film. A modified Ruddlesden-Popper (RP) structure, which closely resembles the atomic structure in the HAADF images, was proposed as an alternate structure for the new model of the BFCO 1/3 unit cell. The refinement of the XRD I-20 result on the BFCO 1/3 film gives a monoclinic unit cell (space group #14, P 2i/n) with lattice parameters of a = 10.9764 A, b = 10.8479 A, and c = 15.9073 A, with a= I= 90° and /3 = 90.3°. The agreement between the simulated and the experimental SAED patterns, however, was limited to the major reflections only.</p> <p>The last investigated oxide is a well known layered perovskite oxide Bi4_xLax Ti30 12 (x = 0.75), BLT, a candidate material likely to replace lead-based ferroelectric materials in various non-volatile memory devices. The TEM investigations focused on the study of the location of the La dopants in the Bi4 Th012 (BiT) parent unit cell so as to provide new information on the good fatigue resistance of BLT. Through HAADF imaging, we showed the presence of translational defects, out-of-phase boundaries (OPBs), which originate from the film-substrate interface. The EELS maps revealed that La ions are most likely to occupy the top part of the Bb02 layer in the BiT parent unit cell structure, instead of the perovskite-like layer. The preferential location of La dopants in the BiT unit cell and the presence of OPBs within the films are discussed in terms of the mechanism of the good fatigue resistance of BLT.</p> <p>In summary, the TEM investigations on several Bi-based perovskite oxides give insights on various structures and defects at atomic scale, which contribute to further understanding of their physical properties. Throughout the study, the use of aberrationcorrectors has demonstrated significant improvement in the quality of TEM imaging and analyses, revealing atomic scale information which was previously unattainable with conventional TEMs.</p>