The five-layered (m = 5) Bi6Ti2.99Fe1.46Mn0.55O18 Aurivillius material is a rare example of a single-phase room temperature ferroelectric-ferrimagnetic multiferroic that shows promise for energy-efficient memory devices. Its ferrimagnetism is thought to derive from the natural partitioning of magnetic ions to the central perovskite layer, engendered by chemically-driven lattice strains, together with ferromagnetic coupling via superexchange mechanisms. Motivated by the expectation of an enhancement in magnetization with increased magnetic ion content, this study examines systematic B-site substitutions with the aim of increasing (from the current level of 40%) the proportion of magnetic ions within the structure. The solubility limits of magnetic cations in this structure and their influence on the superlattice layering are investigated. Studies of Aurivillius phase films on c-sapphire with composition Bi6TixFeyMnzO18 (B6TFMO; x = 2.3 to 3.2, y = 1.2 to 2.0, z = 0.3 to 0.9) demonstrated that above ca. 46% of B-site magnetic cations, the m = 5 structure first rearranges into a mixed-phase material based on m = 5 and six-layered (m = 6) structures and eventually evolves into an m = 6 phase with 54% magnetic cations at the B-site. It is postulated that increasing the number of perovskite layers by forming the m = 6 structure facilitates the accommodation of additional magnetic cations at a lower average manganese oxidation state (+3.3) compared with an equivalent m = 5 stoichiometry (+4.0). While the minor out-of-plane ferroelectric response decreases as expected with increasing structural reorganization towards the m = 6 phase, the predominant in-plane piezoresponse remains unaffected by magnetic cation substitution. This work shows that higher-layered Aurivillius homologues can be synthesized using aliovalent substitution, without requiring epitaxial growth or kinetically constrained methods.
The Aurivillius layer-structures, described by the general formula Bi2O2(Am-1BmO3m+1), are naturally 2-dimensionally nanostructured. They are very flexible frameworks for a wide variety of applications, given that different types of cations can beaccommodated both at the A- and B-sites. In this review article, we describe how the Aurivillius phases are a particularly attractive class of oxides for the design of prospective single phase multiferroic systems for multi-state data storage applications, as they offer the potential to include substantial amounts of magnetic cations within a strongly ferroelectric system. The ability to vary m yields differing numbers of symmetrically distinct B-site locations over which the magnetic cations can be distributed and generates driving forces for cation partitioning and magnetic ordering. We discuss how out-of-phase boundary and stacking fault defects can further influence local stoichiometry and the extent of cation partitioning in these intriguing material systems.
Here, as with previous work, atomic layer deposition (ALD) has been used to deposit Al2O3 on positive electrode active materials, LiCoO2, to create a protective barrier layer, suppress the high potential phase transition, and thus reduce the subsequent Co dissolution. However, in this study it was found that it also resulted in the reduction of the charge transfer resistance at the positive electrode–electrolyte interface, thus enhancing the performance of the battery. Energy-dispersive X-ray spectroscopy, in conjunction with transmission electron microscopy, shows that a discrete Al2O3 shell was not formed under the selected growth conditions and that the Al diffused into the bulk LiCoO2. The resulting active oxide material, which was significantly thicker than the nominally ALD growth rate would predict, is proposed to be of the form LiCoO2:Al with amorphous and crystalline regions depending on the Al content. The cells consisting of the modified electrodes were found to have good cycling stability and discharge capacities of ∼110 mA h g–1 (0.12 mA h cm–2) and ∼35 mA h g–1 (0.04 mA h cm–2) at 50 and 100 C, respectively.
The five-layered (m = 5) Bi6Ti2.99Fe1.46Mn0.55O18 Aurivillius material is a rare example of a single-phase room temperature ferroelectric-ferromagnetic multiferroic that could ideally be suited to future energy-efficient memory devices. This study examines the effect of B-site substitution with the aim of increasing the proportion of magnetic ions within the structure and consequently increasing the saturation magnetisation. Four series of Aurivillius phase films with a target composition of Bi6TixFeyMnzO18 (B6TFMO; x = 2.3 to 3.2, y = 1.2 to 2.0, z = 0.3 to 0.9) were fabricated by chemical solution deposition. Substitution of Ti4+ by Fe3+ and Mn3+ necessitates charge compensation mechanisms and requires accommodation of differing ionic radii. While valence changes of Mn3+ to Mn4+ can act to compensate charge, XRD and TEM analysis is used here to demonstrate that above a threshold of 8 % nominal Mn4+, the m = 5 structure can no longer accommodate the smaller Mn4+ ion and it rearranges into a mixed-phase material based on m = 5 and six-layered (m = 6) inter-growths. Increasing the number of perovskite layers by forming the m = 6 structure facilitates the accommodation of additional magnetic cations at a lower average manganese oxidation state (+3.3). This work provides valuable insight into the design and development of versatile multiferroic phases by describing how the B-site magnetic cation content can be increased to 54 % in m = 6 structures, compared to a solubility limit of 46 % in m = 5 structures.
A series of Aurivillius phase materials,Bi 5 Ti 3-2x Fe 1+x NbxO 15 (x = 0, 0.1, 0.2, 0.3, and 0.4), was fabricated by chemical solution deposition. The effects of aliovalent substitution for the successful inclusion of Fe 3+ and Nb 5+ by replacing Ti 4+ were explored as a potential mechanism for increasing magnetic ion content within the material. The structural, optical, piezoelectric, and magnetic properties of the materials were investigated. It was found that a limit of x = 0.1 was achieved before the appearance of secondary phases as determined by the X-ray diffraction. Absorption in the visible region increased with increasing values of x corresponding to the transition from the valence band to the conduction band of the Fe-eg energy level. Piezoresponse force microscopy measurements demonstrated that the lateral piezoelectric response increased with increasing values of x. Magnetic measurements of Bi 5 Ti 3-2x Fe 1+x NbxO 15 exhibited a weak ferromagnetic response at 2, 150, and 300 K of 2.2, 1.6, and 1.5 emu/cm 3 with Hc of ~40, 36, and 34 Oe, respectively. The remanent magnetization MR of this sample was found to be higher than the range of reported values for the Bi 5 Ti 3 Fe 1 O 15 parent phase. Elemental analysis of this sample by energy-dispersive X-ray analysis did not provide any evidence for the presence of iron-rich secondary phases. However, it is noted that a series of measurements at varying sample volumes and instrument resolutions is still required in order to put a defined confidence levelon the Bi 5 Ti 2.8 Fe 1.1 Nb 0.1 O 15 material being a single-phase multiferroic.
Cobalt doped zinc oxide (ZnO:Co) nanorod arrays with strong visible light absorption were successfully grown via a solution-based method. The deposition technique presented allows rapid (1 h) growth of well aligned nanorods directly onto seed-layer coated substrates at low temperatures (ca. 85 °C), which when compared to previously reported growth methods represents a significant improvement in terms of routes to the production of visible-light absorbing ZnO-based materials. Changing the cobalt concentration in the growth solution allows the controlled growth of ZnO:Co nanorods with variable visible light absorption. The emergence of strong additional, cobalt 3d related visible light absorption features and band gap narrowing with increasing concentration of cobalt (until 20% cobalt/zinc concentration in growth solution) is demonstrated. A cobalt concentration of up to 2.2 atom% (at 30% cobalt/zinc concentration in growth solution) can be achieved and careful analysis of the crystal growth process and material properties of the nanorod arrays shows that Co2+ successfully replaces Zn2+ in the lattice.
An abstract is not available for this content so a preview has been provided. As you have access to this content, a full PDF is available via the ‘Save PDF’ action button.
'/%3e%3cpath d='M200 752.362h200v300H200z' style='fill:%23fff;fill-opacity:1;stroke:none' transform='translate(0 -752.362)'/%3e%3cpath d='M400 752.362h200v300H400z' style='fill:%23ff883e;fill-opacity:1;stroke:none' transform='translate(0 -752.362)'/%3e%3c/svg%3e)
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
