Nanocomposite oxide ceramics have potential uses as ferroelectrics, fast ion conductors, nuclear fuels and for storing nuclear waste, generating a great deal of scientific interest on the structure, properties, and applications of these blended materials. At the Los Alamos National Laboratory in Los Alamos, N.M., researchers have uncovered new properties in nanocomposite oxide ceramics. “The interfaces separating the different crystalline regions determine the transport, electrical, and radiation properties of the material as a whole,” said Pratik Dholabhai, principal researcher on the project. “It is in the chemical makeup of these interfaces where we can improve features such as tolerance against radiation damage and fast ion conduction.”

Composite Attributes

A composite is a material containing grains, or chunks, of several different materials. In a nanocomposite, the size of each of these grains is on the order of nanometers, roughly 1,000 times smaller than the width of a human hair. In the context of nuclear energy, composites have been proposed for the fuel itself, as a way to improve the basic properties of the material—such as the thermal conductivity, for example. It is the thermal conductivity that dictates how efficiently energy can be extracted from the fuel. Composites have also been created to store the by-products of the nuclear energy cycle (i.e., nuclear waste), where the different components of the composite can each store a different part of the waste.

However, composites have much broader applications. The interfaces provide regions of unique electronic and ionic properties and have been studied for enhanced conductivity for applications related to batteries and fuel cells.

Misfit Dislocation Discoveries

Using simulations that explicitly account for the position of each atom within the material, the Los Alamos research team examined the interface between strontium titanate (SrTiO₃) and magnesium oxide (MgO), demonstrating for the first time a strong dependence of the dislocation structure at oxide heterointerfaces on the termination chemistry. SrTiO₃ can be viewed like a layer cake, with alternating planes of strontium oxide (SrO) and titanium dioxide (TiO₂). Thus, in principle, when matching SrTiO₃ with another material, there is a choice as to which layer is in contact with the other material.  The simulations reveal that SrO- and
TiO₂-terminated interfaces exhibit remarkably different atomic structures. These structures, characterized by so-called misfit dislocations that form when the two materials do not exactly match in size, dictate the functional properties of the interface, such as the conductivity.

“When marrying different compounds to form composites, the interface structures are determined by a number of factors, such as the orientation relationship between the two materials,” said Blas Uberuaga, lead researcher on the effort. “This work indicates that the termination chemistry, which is independent of the orientation relationship and which can in principle be varied significantly for a given oxide, is another key parameter determining interfacial structure and the corresponding interfacial properties.”

Future Directions

The observed relationship between the termination chemistry and the dislocation structure of the interface offers potential avenues for tailoring transport properties and radiation damage resistance of oxide nanocomposites by controlling the termination chemistry at the interface. This could lead to new functional materials in a number of technological areas.

“Our results show that the misfit dislocation structure at oxide/oxide interfaces is very sensitive to the termination chemistry at the interface,” said Dholabhai. “This has important ramifications beyond the radiation damage applications that motivated our study. For example, similar interfaces are studied for their exotic electronic and ferroelastomagnetic properties. We suspect that these properties will themselves be very sensitive to the misfit dislocation structure and should lead to new avenues for designing advanced electronic materials as well.”

The research is described in a paper in Nature Communications entitled “Termination Chemistry-Driven Dislocation Structure at SrTiO₃/MgO Heterointerfaces.” The work was funded by the Center for Materials at Irradiation and Mechanical Extremes (CMIME), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under the Award Number 2008LANL1026.

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