The depletion of fossil energy and the deterioration of the environment have led to increased focus on the development of clean energy for sustainable use. Thermoelectric materials are proposed to contribute towards more sustainable energy. Ideally, the lattice of a thermoelectric material would only impede the movement of the phonons, resulting in the desired low thermal conductivity, and let the electrons move freely, yielding the required high electrical conductivity. Such materials were coined phonon glass, electron crystal (PGEC) materials by Slack. Composite structures, i.e. materials considering of two different structures with different repeat units, may very well constitute the epitome of PGEC materials. The new telluride Tl2Ag12Te7+δ adopts an incommensurately modulated crystal structure. The two parts of this composite structure are i) a covalent three-dimensional network of the composition Tl2Ag12Te6, and ii) a distorted linear Te atom chain. The distortion of the latter strongly impacts its direct neighbors of the Tl2Ag12Te6 network, thereby contributing to the desired low thermal conductivity. As this material consists of atoms in low valence states and with small electronegativity differences, the electrons may have high carrier mobility. As a consequence, Tl2Ag12Te7+δ represents the first misfit material with a thermoelectric figure-of-merit above 1, ultimately supporting the concept of using composite structures in thermoelectrics.

Ultralow thermal conductivity and high thermoelectric performance in a new composite structure

2019 Spring National Meeting

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