Order-Disorder Transition in Inorganic Clathrates Controls Electrical Transport Properties

Inorganic clathrates have been extensively investigated owing to their unique and intriguing atomic structure as well as their potential as thermoelectric materials. The connection between the chemical ordering and the physical properties has, however, remained elusive. Here, this relation is uncovered through a combination of first-principles calculations, atomistic simulations, and experimental measurements of thermodynamic as well as electrical transport properties. This approach is, specifically, used to reveal the existence of an order-disorder transition in the quaternary clathrate series Ba8AlxGa16-xGe30. The results, furthermore, demonstrate that this phenomenon is responsible for the discontinuity in the heat capacity that has been observed previously. Moreover, the unusual temperature dependence of both Seebeck coefficient and electrical conductivity can be fully explained by the alterations of the band structure brought about by the phase transformation. It is finally argued that the phenomenology described here is not limited to this particular material but should be present in a wide range of inorganic clathrates and could even be observed in other materials that exhibit chemical ordering on at least one sublattice.

Automated summary

By combining first-principles calculations, atomistic simulations, and experimental measurements, the connection between chemical ordering and physical properties in inorganic clathrates has been revealed. An order-disorder transition in the quaternary clathrate series was discovered, leading to discontinuity in heat capacity and explaining the unusual temperature dependence of Seebeck coefficient and electrical conductivity. It is suggested that this phenomenon may not be limited to specific materials but could be observed in a wide range of inorganic clathrates and materials with chemical ordering on sublattices.