ICAMS / Interdisciplinary Centre for Advanced Materials Simulation

Silicon based thermoelectric materials: a high throughput combinatorial search

Waste heat is one of the main sources for energy loss. Thermoelectrics may be used for an efficient conversion of this waste heat, provided suitable materials are available. For practical use materials with a high thermoelectric figure of merit (zT>1) are required. While several materials with a high value of zT are available, their practical use is often limited by cost or toxicity. One of the interesting material classes for the search of new thermoelectric materials are silicides, which benefit from the abundance and non-toxity of Si. Materials like MnSi1.73, ReSi1.75 and the Mg2(Si1-x(IV)x) solid solutions have received attention for their thermoelectric properties.

In this project the phase diagrams and transport properties of silicides are systematically investigated using our recently developed high-throughput environment (HTE). The Seebeck coefficient can be calculated from the electronic structure and it therefore possible to screen compounds both with respect to phase stability and transport properties. By a simple argument it can be shown that a zT>1 requires S>156 μV/K.

A high-throughput screening of binary 3d, 4d and 5d transition metal silicides shows that semiconducting phases are exclusively found on the silicon rich side of the group 6-9 transition metal silicides. We identify several candidates with potential for thermoelectric applications. This includes known thermoelectrics like Mn4Si7, FeSi2, CrSi2 or Ru2Si3, as well as new potentially meta stable materials like Rh3Si5, Fe2Si3 and an orthorhombic CrSi2 phase. Calculated transport properties are in good agreement with available experimental data at the respective carrier concentration. The calculations also show that a better thermoelectric performance may be achieved upon optimal doping.

Calculated formation energy Ef of group 6-9 transition metal silicides. The calculated convex hull is shown as blue line. Alloys within 50 meV/atom from the convex hull are additionally highlighted with blue squares (open squares: metals, filled squares: semiconductors).

The high throughput computational approach is also used to systematically investigate the phase diagrams of ternary and quaternary Mg2(Si1-x-y(IV)xAy) alloys. In the compound Mg2(Si1-x-y(IV)xAy), IV is a main group IV element and A is main group III-V element which we imagine controls the carrier concentration. Consequently, Mg2(Si1-x-y(Sn)xSby) would be a n-type conductor and Mg2(Si1-x-y(Sn)xIny) would be a n-type conductor. A high-throughput approach will be necessary for a full understanding of the Mg2(Si1-x-y(IV)xAy) system. Firstly, in order to map out the limits of solid solubility and the compositions where new phases are formed. Secondly, the thermoelectric properties are highly dependent on the detailed carrier and impurity concentration and it is thus not possible to draw general conclusions about the Mg2Si system by studying only a few stoichiometries.

The high-throughput approach will allow for establishing a phenomenological connection between structure and transport properties or phase stabilities. The connections will be rationalized in terms of simplified models of the electronic structure.

The project will be based on and contribute to the development of a common computational infrastructure at ICAMS, including a database of DFT calculations for future data-mining projects.

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