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Trinity College Dublin

Novel Materials for the future

Computational design of functional complex oxides

Dr. Claude Ederer, School of Physics, Trinity College Dublin

Complex transition metal oxides are fascinating materials both from a basic science perspective as well as from a technological viewpoint (see Figure 1 for some examples). The unique electronic structure of these materials leads to a very strong coupling between structural, electronic, and magnetic properties. To understand this complex interplay between the various degrees of freedom is a great challenge for modern condensed matter physics. In addition, the same interplay leads to a tremendous variety of physical properties, such as for example metallic, semiconducting, or insulating behavior, high dielectric constants, piezoelectricity, ferroelectricity, ferromagnetism, metal-insulator transitions, and high temperature superconductivity. These functional properties are extremely attractive for use in modern electronic devices such as nonvolatile memory, integrated circuits, or new types of sensors and actuators, and in addition offer great prospects for the development of future technologies [1].

The unique features of transition metal oxides are a result of the partial occupation of the electronic d shell on the transition metal cation. The corresponding electronic orbitals are intermediate between being "atomic-like" and "band-like", i.e. between being localized at the cation sites and being delocalized throughout the crystal by the formation of covalent chemical bonds. The extent to which these d electrons form chemical bonds crucially depends on volume, transition metal--oxygen bond length, bond angle (the angle formed by a transition metal--oxygen--transition metal structural unit), and the symmetry of the "crystal-field", which is caused by the anion polyhedra surrounding the metal cation. One very common and probably the most studied crystal structure of complex transition metal oxides, the perovskite structure, is depicted in Figure 2. In the perovskite structure the transition metal cations form a simple cubic lattice and are surrounded by octahedra of oxygen anions; additional (non-transition metal) cations occupy the space in between these octahedra. Small distortions from the perfect perovskite structure or small changes in volume or electron count (i.e. doping) can tip the balance toward either more localized or more covalent, thereby eventually changing the system from insulating to metallic or fundamentally affecting the degree and overall character of the magnetic ordering.

    Figure 1: Crystal structures of some complex transition metal oxides. Rutile VO2 (top left) exhibits a metal-insulator transition coupled to a structural distortion. The spinel structure (general chemical composition AB2O4, top right) is adopted by many magnetic oxides. TbMn2O5 (bottom left) and YMnO3 (bottom right) are examples of multiferroic materials that exhibit ferroelectricity simultaneously with magnetic order.

  Figure 2: Ideal cubic perovskite structure (left) and distorted perovskite structure (right), as found for example in LaMnO3. The transition metal cations (not shown) are located in the centers of octahedra made out of oxygen anions; the intermediate space is filled with additional (generally non-transition metal) cations. The small tilting of the oxygen octahedra can lead to dramatically different physical properties.

The same strong coupling between structural, electronic, and magnetic degrees of freedom that result in the fascinating spread of properties in complex transition metal oxides on the other hand also represents an obstacle for the accurate experimental synthesis and characterization of these materials. Small changes in composition or the accidental presence of impurities or other defects can alter the properties significantly. It is therefore very desirable to obtain reference data via atomistic modeling of these systems. In particular, atomistic modeling based on density functional theory at various levels of sophistication has been proven to give quantitatively reliable results for many characteristic properties of complex oxides (see e.g. Ref. [2]). Such first principles calculations neither rely on simplified model assumptions nor include any empirical parameters that would have to be fitted to experimental data. By predicting the physical properties of transition metal oxides for an ideal system free from structural defects and impurities these calculations can therefore guide the experimental research efforts towards the most promising materials systems for future device applications. In addition, since the results of these computations can be analyzed in terms of the quantum mechanical wave-function, they also provide means for obtaining an improved understanding of the underlying physics. This makes first principles calculations an invaluable tool for the rational design of new materials and novel functionalities.

We are currently building up a new group at TCD which combines the development of new computational tools for a quantitative (i.e. predictive) description of complex transition metal oxides with a very pronounced materials research effort that attempts to design novel materials for future technologies by seeking an improved understanding of the physics behind their functional properties. An area of particular interest is the study of multifunctional complex oxide hetero-structures. Such nano-composites made out of different functional oxides can exhibit rather unexpected and unconventional properties. As an example, it has recently been shown that the interface between two different complex oxides can become metallic, even though the two constituent materials themselves are insulating [3]. The two-dimensional electron gas that forms at the interface of these systems exhibits electron densities and mobilities that are orders of magnitudes larger than in conventional semiconductor devices. In another example we have recently shown that in thin films of so-called multiferroic complex oxides, materials that simultaneously exhibit ferroelectric and magnetic order, it is possible to switch the magnetic domains using an electric field [3]; an effect which opens up entirely new perspectives for digital memory applications.

Recent progress in thin film fabrication together with advanced computational modeling nowadays facilitates the rational design of atomic scale devices with formerly unachievable properties. Complex transition metal oxides are extremely attractive for this purpose due to their great chemical and structural flexibility. Figure 1 offers just a glimpse at the different ways how in these systems various types of polyhedra can be stacked together to form one-, two-, or three-dimensional networks, thereby giving rise to a multitude of functional properties. The possibility to grow nano-composites from these oxides allows for even more flexibility. Computational studies using first principles techniques in combination with more simplified models are an important ingredient in the design process of these novel materials for future technologies.

Acknowledgments
Our future research activities are funded by the Science Foundation Ireland through a President of Ireland Young Researcher Award.

References

  1. S.-W. Cheong, Nature Materials 6, 927 (2007).
  2. C. Ederer and N. A. Spaldin, Current Opinion in Solid State and Materials Science 9, 128 (2005).
  3. A. Ohtomo and H. Hwang, Nature 427, 423 (2004).
  4. T. Zhao {\it et al.}, Nature Materials 5, 823 (2006).

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