Synthesis and characterisation of Aurivillius phase materials

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Halpin, Jennifer
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University College Cork
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Multiferroic materials, which are materials that exhibit two or more ‘ferro’ orders in a single phase, such as both ferroelectricity and ferromagnetism in the same phase, are an attractive area of research due to their novel properties and potential uses. Emerging areas of research for multiferroic materials include the biomedical, photocatalytic and photovoltaic areas, four state memory devices and multiferroic tunnel junctions. Despite the increasing research interest there is currently a lack of genuine single-phase multiferroic materials. Ferroelectricity and ferromagnetism arise from fundamentally different conditions, where ferroelectricity requires ions with empty orbitals to accept electron density donation and ferromagnetism requires partially filled orbitals to allow unpaired electrons to align to give net spin. It is therefore necessary to have materials capable of supporting different types of ions to provide the conditions necessary for both ferroelectricity and ferromagnetism. The Aurivillius phase materials (Bi2O2(Am-1BmO3m+1) m = 1-9) have a naturally layered structure capable of accommodating a range of ions at the A and B-sites of the perovskite units. The Aurivillius materials are well known ferroelectrics with the electrical polarisation originating from polar distortions (rigid shifts of the Bi2O2 layers, displacements of the A-site / B-site cations) and tilts and rotations of the BO6 unit. Substitution of magnetic ions such as iron and manganese at the B-sites has been shown to produce room temperature multiferroic materials. Magnetic coupling between ions requires proximity between the magnetic ions. The work presented in this thesis involves an investigation into two approaches for improving the magnetic behaviour of the Aurivillius phase materials. The first approach used is to increase the concentration of magnetic ions in the material, which should increase the likelihood that the magnetic ions will be located close to each other within the crystal lattice. The second approach used is to locally concentrate the magnetic ions within the structure without the need to increase the total concentration of the ions. To increase the concentration of magnetic ions in the m = 4 Aurivillius material a method known as aliovalent substitution was employed. The co-substitution of Fe3+ and Nb5+ for Ti4+ was performed to maintain a material with a net neutral charge. For the material Bi5Ti3-2xFe1+xNbxO15 (x = 0, 0.1, 0.2, 0.3 and 0.4) a solubility limit of x = 0.1 was found before the appearance of secondary impurity phases became apparent. The increased iron content resulted in an increased absorption in the visible light region of the solar spectrum. It was found that there is generally a larger piezoelectric response for materials with more iron due to the increased distortion of the perovskite unit due to the substitution of the larger ions. Magnetic measurement of the x = 0.1 material demonstrated ferromagnetic behaviour with remnant magnetisation of MR = 1.5 emu/cm3 at 300 K. Previous work of the Bi6Ti2.8Fe1.52Mn0.68O18 material deposited using a very similar process exhibited a magnetic response of MR = 0.18 emu/cm3 [1]. A systematic study into the m = 5 Bi6TixFeyMnzO18 Aurivillius materials established a threshold of 52 % Ti occupancy at the B-site of the perovskite unit below which the structure undergoes rearrangement to the higher m = 6 phase. The driving force behind this change is the net charge that results from the substitution of Fe3+ and Mn3+ for Ti4+. It is proposed that the structure responds with oxidation of Mn3+ to Mn4+ and rearrangement to the higher ‘m’ phase to accommodate a charge-balanced material. Detailed transmission electron microscopy analysis confirms the presence of mixed Aurivillius phases. To examine the second approach outlined above, a modified direct liquid injection chemical vapour deposition (DLI-CVD) method called sequential DLI-CVD was employed. With this method, gaseous precursors enter the reaction chamber in the order that mimics the Aurivillius structure. In this way it was hoped to impose order on the location of the ions in the structure. Using this method, Bi5Ti2.9Fe1.1O15 (BTFO) and Bi5Ti2.8Fe1.1Mn0.1O15 (BTFMO) films were deposited on epitaxial substrates (LSAT and STO) at 710 °C. The as-deposited crystalline thin films displayed a magnetic response despite the relatively low concentration of magnetic ions in the material (30 % Fe and Mn in BTFMO compared to 44 % of Fe and Mn in previous B6TFMO room temperature multiferroic material [2]). The influence of the strain applied by the substrate was examined for its effect on the structure and magnetic behaviour of the materials. A larger magnetic response was observed for materials deposited on STO than on LSAT substrates (at 20 K, BTFMO on STO MR = 3.51 emu/cm3 and BTFMO on LSAT MR = 1.50 emu/cm3). Further work found that crystalline Aurivillius structures were formed on epitaxial substrates at temperatures as low at 610 °C. A further modification of the sequential DLI CVD recipe was introduced which was found to help maintain the levels of bismuth within the materials throughout the growth process. Narrower FWHM of the peaks obtained during XRD analysis suggest that fewer dislocations due to the formation of different ‘m’ phases were occurring using the method. This modified sequential DLI-CVD method involved an injection of the bismuth precursor accompanying each precursor injection during the deposition. Finally, the work presented here is summarised and some suggestion for future work are presented. The work presented in this thesis has advanced the understating of the Aurivillius phase materials and their potential as multiferroic materials. In particular, the in-depth analysis of their structural response to changing chemical composition and the development of experimental techniques to locally concentrate magnetic ions within the Aurivillius structure will be critical to their future use in the area of multiferroic research. References [1] L. Keeney et al., "Magnetic Field-Induced Ferroelectric Switching in Multiferroic Aurivillius Phase Thin Films at Room Temperature," Journal of the American Ceramic Society, vol. 96, no. 8, pp. 2339-2357, 2013/08/01 2013, doi: 10.1111/jace.12467. [2] L. Keeney, C. Downing, M. Schmidt, M. E. Pemble, V. Nicolosi, and R. W. Whatmore, "Direct atomic scale determination of magnetic ion partition in a room temperature multiferroic material," Scientific Reports, vol. 7, May 2017, Art no. 1737, doi: 10.1038/s41598-017-01902-1.
Chemistry , Multiferroic , Ferroelectric , Ferromagnetic , Aurivillius phase materials
Halpin, J. C. 2021 Synthesis and characterisation of Aurivillius phase materials. PhD Thesis, University College Cork.
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