Controlling structural complexity and dynamics in dicyanometallates

Thermoelectric materials transform heat into electricity directly. These materials could revolutionise energy efficiency and solve problems associated with the world's growing energy consumption. Their reliability, scalability and long lifespan offer huge potential. It is their low efficiency that has prevented their widespread use. A good thermoelectric material conducts electricity well but heat poorly. Unfortunately, this combination of properties is rare.

This research aims to further design of thermoelectrics by probing ways of reducing thermal (or heat) conductivity whilst not affecting other properties of the material. This research is therefore concerned with altering the atomic vibrations of a material. Atoms in solids are constantly vibrating. These vibrations are concerted between atoms. Scientists can calculate the energy of the vibration and the associated movement of atoms. Phonons can be measured using different techniques such as shining infrared light or neutrons on a material. Phonons are critical to understand the thermal properties of a material. Properties such as the expansion of a material as it is heated or its thermal conductivity all rely on phonons. Phonons are the heat carriers for solid materials, and disrupting them leads to lower thermal conductivity.

An intuitive way of reducing the thermal conductivity is to introduce defects into the material. Defects are imperfections within the material - this can often be a gap where there should be an atom in the material. Or even an atom of the material in the wrong position. Defects break up the atomic vibrations and can 'scatter' the phonons. Unfortunately, defects also tend to scatter electrons reducing the electrical conductivity of the material. Yet theoretically, one could introduce disorder into a material that can affect the thermal conductivity much more than the electrical conductivity. For this to occur, the disorder must be not be random but correlated. The interplay between disorder and phonons is the subject of this research. This research aims to answer the fundamental question: can one introduce disorder in a systematic manner to affect certain phonons?

Dicyanometallates are used to answer this question. These form large crystals and can have their phonons measured easily using neutrons. They also allow a straightforward mechanism for controllably introducing disorder. The hope is that the research proceeds via an iterative process. The effect of disorder on phonon properties will be measured experimentally. These results will then be used to predict the effects of disorder on phonon properties using computational methods. This has the potential to greatly aid thermoelectric design.

This research falls in the EPSRC Physical Sciences Research Area. Work will be carried out from the Goodwin and Deringer groups.

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