We propose to study the room temperature crystal structure of Li-doped Mg2Si-based solid solutions, which represent one of the most promising material classes for mid-temperature range thermoelectric applications. We plan to investigate p-type samples which exhibit far inferior thermoelectric properties when compared to their n-type counterparts, in part related to low charge carrier concentrations, even though nominal carrier concentrations are quite high and high thermoelectric performance should be observed. The determination of the crystal structure will give us insight into the distribution of the Li dopant, the formation of Mg-defects and/or the occupancy of interstitial positions. We ask for 3 days of beam time.