Beautiful forms of ferromagnetic fluid. Iron dissolved in a liquid under the influence of a magnetic field. (Image Credits: LuYago/shutterstock.com)
Complex oxides are a branch of metal oxides that have shown some of the most promising properties across all metal oxides. Whilst each structure is different, there are always certain properties that are exhibited by a complex oxide, but they do vary from material to material. In this article, we look at what complex oxides are, and the common properties exhibited by these materials.
What are Complex Oxides
Many will be familiar with metal oxides, but within this general class is a specific category known as complex oxides. Metal oxides, as a general class, are inorganic materials that contain both oxygen and metal ions which are often crystalline in nature and possess solid-state lattices. There are two instances where metal oxides can be classed as a complex oxide. The first is where the material contains oxygen and at least two different metallic elements, and the second is when the material contains oxygen and one metallic element that is in two or more oxidation states within the lattice.
There are many different properties associated with complex oxide materials, including ferromagnetism, ferroelectricity, piezoelectricity, and high-temperature superconductivity. But these properties are not restricted to just complex oxides, yet at least one is exhibited by all complex oxide materials. These properties are often exhibited because the electrons within the d and f orbitals are strongly correlated, and this means that each electron has an individual influence on the neighboring electrons. This electronic arrangement causes the materials to be a hybrid electronic structure that lies between ionic and free-electron materials.
Given by how complex oxides are defined, there are a significant amount of materials that fall into this category, from iron-based ferromagnetic materials to superconducting lanthanum barium copper oxides (LBCOs), lanthanum strontium manganite oxides (LSMOs), wurtzite materials, and perovskites, to name a few. Recent years has seen these materials expand from the conventional metal oxides seen in inorganic chemistry and now include materials which are multiferroic, i.e. they exhibit both ferromagnetic and ferroelectric properties, and 1D ferroelectric nanostructures.
Given that there are so many different types of complex oxides, all of which exhibit different properties depending on the structural composition and arrangement, we’re going to focus on the general properties exhibited by the whole class of complex oxides, rather than individual materials.
Ferromagnetism is a property often seen with iron-based complex oxides but is present with other transition metal oxide complexes. Ferromagnetism is the mechanism by which materials become a permanent magnet. One of the fundamental principles of electrons is that they have a magnetic dipole moment which can produce a small magnetic field and be aligned. Complex oxides that exhibit ferromagnetism also exhibit long-range ordering at the atomic level and this causes the unpaired electron spins to line up in parallel to each other when there is an applied magnetic field, which in turn causes the material to possess an internal magnetization.
Ferroelectricity is a property seen with dielectric (insulating) complex oxides and it is when the material exhibits spontaneous electrical polarization. This is caused by the separation of the positive and negative charges within the lattice leading to one side of the material becoming positively charged, and the other side becoming negatively charged. Just like in ferromagnetism, there are small localized dipoles within the lattice, but the positive and negative aspects of the dipole are slightly separated. So, when an electric field is applied to the material, the different charges of the dipole line up into clusters which generates a separation of charges throughout the complex oxide and causes a temporary polarization.
Piezoelectricity is the ability for a complex oxide to deform and generate an electric current. This is a common property for many solid-state crystalline materials, and there are instances where complex oxides fall into this category. A piezoelectric current is generated by these materials because the lattice deforms at the atomic level under applied stress. This causes the ions within the lattice to move from their normal location within the lattice, and this, in turn, causes like charges to become closer together. This then alters the charge balance within the lattice, and the like charges gather at either end of the material, which generates an external electric field and a voltage through the material.
Superconductivity is a phenomenon that occurs in complex oxides and is categorized by the material having an electrical resistance exactly equal to zero. It also means that superconducting materials can maintain a current without an applied voltage—and this can last for years without degradation occurring. Superconductivity also brings about other effects, such as the Meissner effect, which is when the magnetic field is ejected from the material—and this causes magnetic materials to levitate above the superconducting material. Superconductivity can be found in many materials, but often, they have to be cooled to almost absolute zero. Many complex oxides are classed as high-temperature superconductors, and this is when the superconducting effects are realized at temperatures above 77 K.
Sources and Further Reading
- American Physical Society: http://meetings.aps.org/Meeting/MAR16/Event/266959
- Tsinghua University: http://info.phys.tsinghua.edu.cn/yupu/research.html
- Lawrence Berkeley National Laboratory: https://commons.lbl.gov/display/neatongroup/Complex+Oxides
- “Ferroelectric thin films: Review of materials, properties, and applications”- Setter N. et al, Journal of Applied Physics, 2006, DOI: 10.1063/1.2336999
- “Magnetoelectric Effects in Complex Oxides with Competing Ground States”- Vaz C. A. F. et al, Advanced Materials, 2009, DOI: 10.1002/adma.200900278
- “Tuning the properties of complex transparent conducting oxides: role of crystal symmetry, chemical composition and carrier generation”- Medvedeva J. E. and Hettiarachchi C. L, Physical Review B, 2010, DOI: 10.1103/PhysRevB.81.125116
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- “The electronic properties of complex oxides of bismuth with the mullite structure”- MacKenzie K. J. D., Journal of the European Ceramic Society, 2008, DOI: 10.1016/j.jeurceramsoc.2007.03.012