Effects of Oxygen on Lattice Defects in Single-Crystalline Mg(2)Si Thermoelectrics

Lattice defect engineering has attracted attention due to its ability to develop thermoelectric materials with low thermal conductivity. For Mg(2)Si single crystals (SCs), Si vacancy (V(Si)) defects can be introduced and consequently result in the formation of dislocation cores. These lattice defects confer Mg(2)Si SCs with a lower thermal conductivity compared to Mg(2)Si polycrystals. To reveal a mechanism for the stabilisation of V(Si) in the Mg(2)Si 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 Mg(2)Si SCs. However, O was suggested to be located not at the specific site in the crystal lattice of Mg(2)Si but at dislocation cores. The interaction between O and the dislocation cores in the Mg(2)Si SC is expected to immobilise dislocation cores, leading to the stabilisation of V(Si) formation.