Oxidation of half-Heusler NiTiSn materials: Implications for thermoelectric applications

Oxidation of phases such as NiTiSn is critical for understanding the mechanisms involved during the experimental synthesis and may also have an influence on the thermoelectric properties of this half-Heusler phase. Indeed, oxygen may dope the phase by entering in its structure or generate secondary oxide phases which may improve or decrease the global figure of merit of the sample. In this study, an ab initio method is used to predict the behavior of the NiTiSn phase in the presence of oxygen. For that, the thermodynamic database containing enthalpies of formation at 0 K of the intermediate phases of the Ni-O-Sn-Ti system is calculated. This base will allow us to predict multiphased regions as a function of the composition of an alloy. The enthalpy of formation of the NiTiSn phases containing oxygen atoms is then compared to the enthalpy of formation of the multiphased regions with the same composition. Using this methodology, thermodynamics can predict if oxygen will enter the structure or if a multiphased alloy is thermodynamically more favorable. Results show that oxygen will decompose the NiTiSn half-Heusler into titanium oxide and nickel stannides. On the basis of the literature review, it seems that the presence of metallic titanium oxides will decrease the Seebeck coefficient and the thermal conductivity but will highly improve the electrical conductivity. Finally, for a moderate amount of oxygen, the figure-of-merit of the sample may be globally unchanged.