Effects of Oxygen on Lattice Defects in Single-Crystalline Mg₂Si Thermoelectrics

Lattice defect engineering has attracted attention due to its ability to develop thermoelectric materials with low thermal conductivity. For Mg₂Si single crystals (SCs), Si vacancy (V_Si) defects can be introduced and consequently result in the formation of dislocation cores. These lattice defects confer Mg₂Si SCs with a lower thermal conductivity compared to Mg₂Si polycrystals. To reveal a mechanism for the stabilisation of V_Si in the Mg₂Si SCs, we investigated the effects of oxygen (O) on lattice defects by performing electronic structure calculations, secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and photoelectron holography. On the basis of these calculations, we predicted that O stabilised the formation of V_Si when it was located at the Si site or at an interstitial site. All experiments confirmed the presence of O inside the Mg₂Si SCs. However, O was suggested to be located not at the specific site in the crystal lattice of Mg₂Si but at dislocation cores. The interaction between O and the dislocation cores in the Mg₂Si SC is expected to immobilise dislocation cores, leading to the stabilisation of V_Si formation.