Unravelling the interplay between defects, dopant species, and thermoelectric properties in p-type Mg2X (X=Si, Ge, Sn)

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Thermoelectric materials can convert waste or process heat directly into usable electrical energy. They are therefore a fascinating opportunity to increase the energy efficiency of various industrial processes and thus reduce fossil fuel consumption. For the temperature range of 500 K to 800 K - where a large fraction of the reusable heat is available - n-type magnesium silicide based solid solutions Mg2X (X = Si, Ge, Sn) are among the most promising thermoelectric materials. However, for the corresponding p-type solid solutions the thermoelectric properties are experimentally found to be far inferior and fail to meet the theoretical expectations. Crystal structure defects are ubiquitous in Mg2X and our analysis of the available experimental data and recent theoretical studies indicates that these defects control the thermoelectric properties of p-type Mg2X to a large degree. The aim of the proposed work is therefore to elucidate the effect of defects on the thermoelectric properties of p-type Mg2X. We will employ controlled variation of material/synthesis parameter and microstructural analysis to establish correlations between experimentally accessible parameters and defect concentration. From accompanying transport measurements the effect of defects on the thermoelectric properties shall be quantified. Semi-analytical modelling of the thermoelectric properties including the effect of defects will guide the way for further experimental material improvement.