Dynamically Disordered Solid Materials Described with Theoretical Models

Solid materials are actually not quite as solid as they seem. Generally, each atom usually vibrates around a specific position in the material. Most theoretical models that aim to define solid materials are based on the notion that the atoms keep their positions and do not travel great distances from them.

This is not the case for some materials, such as materials with very high ionic conductivity and those where the building blocks are not only atoms but also molecules. Several of the perovskites that are promising materials for solar cells are of this kind.

Johan Klarbring, Doctoral Student in Theoretical Physics, Linköping University.

Perovskites are identified by their crystal structures and come in various forms. Their elements can be both molecules and atoms. The atoms in the molecules vibrate, but the total molecule can also rotate, which means that the atoms travel considerably more than is frequently presumed in the calculations.

Dynamically disordered solid materials

Materials that display this atypical behavior are called “dynamically disordered solid materials”. Dynamically disordered solid materials exhibit enormous potential in environmentally sensitive applications. Materials that are good ionic conductors show, for instance, potential in the development of solid electrolytes for fuel cells and batteries, and for thermoelectric applications.

Nevertheless, the properties of materials have been complicated to calculate theoretically and scientists have repeatedly been forced to use laborious experiments.

Jonas Klarbring has formulated a computational technique that describes accurately what takes place when these kinds of material are heated and experience phase transitions. Johan Klarbring and his supervisor, Professor Sergei Simak, have reported the outcomes in the scientific journal Physical Review Letters.

Bismuth oxide

They have examined bismuth oxide (Bi₂O₃), a material said to be an excellent ionic conductor. This oxide, where current is conducted by oxide ions, is the finest oxide ion conductor of all identified solid materials. Experiments have revealed that it has a low conductivity at low temperatures, but when heated it experiences a phase transition into a dynamically disordered phase having high ionic conductivity.

The article in Physical Review Letters describes how we have been able for the first time to theoretically describe the phase transition in bismuth oxide, and calculate the temperature at which it occurs. This provides an important theoretical basis for the development of, for example, electrolytes in fuel cells, where it is important to know exactly when the phase transition takes place. I start from an ordered phase, which is well-described by conventional methods. I then use a technique known as ‘thermodynamic integration’, which I have adapted to deal with the disordered motion. The ordered phase is coupled to the disordered one, with the aid of a series of quantum mechanical calculations, carried out at the National Supercomputer Centre at LiU.

Johan Klarbring, Doctoral Student in Theoretical Physics, Linköping University.

Perovskites next

The theoretical calculations are in complete agreement with how the material acts in laboratory experiments.

The scientists currently plan to test the technique on other stimulating materials, such as perovskites, and on materials having high lithium ionic conductivity. The latter are of interest for the progress of high-performance batteries.

“Once we have a deep theoretical understanding of the materials, it improves the possibilities to optimise them for specific applications”, concludes Johan Klarbring.

The study is supported by the Swedish Research Council and the Swedish Government Strategic Research Area initiative in Material Science on Functional Materials at Linköping University.