Temperature Dependent n-Type Self Doping in Nominally 19-Electron Half-Heusler Thermoelectric Materials

The discovery of a semiconducting ground state X_yYZ (y = 0.8 or 0.75) in nominally 19-electron half-Heusler materials warrants a closer look at their apparently metallic properties that often make them good thermoelectric (TE) materials. By systematically investigating the temperature dependence of off-stoichiometry (x) in V_{0.8+ x}CoSb, Nb_{0.8+ x}CoSb, and Ti_{0.75+ x}NiSb it is found that x invariably increases with increasing temperature, leading to an n-type self-doping behavior. In addition, there is also a large phase width (range of x) associated with each phase that is temperature dependent. Thus, unlike in typical 18-electron half-Heuslers (e,g, TiNiSn), the temperature dependence of vacancy and carrier concentration (n) in nominally 19-electron half-Heuslers links its transport properties to synthesis conditions. The temperature dependence of x and n are understood using density functional theory based defect energies (E_d) and phase diagrams. E_d are calculated for 21 systems which can be used in predicting solubility in this family of compounds. Using this simple strategy, suitable composition and temperature synthesis conditions are devised for obtaining an optimized n to engineer TE properties in phase-pure V_{0.8+ x}CoSb, and the previously unexplored Ta_{0.8+ x}CoSb.