Thermoelectric devices, which allow direct conversion of heat into electrical energy, require materials with improved figures of merit (

z T

) in order to ensure widespread adoption. Several techniques have been proposed to increase the

z T

of known thermoelectric materials through the reduction of thermal conductivity, including heavy atom substitution, grain size reduction and inclusion of a semicoherent second phase. The goal in these approaches is to reduce thermal conductivity through phonon scattering without modifying the electronic properties. In this work, we demonstrate that Ni interstitials in the half-Heusler thermoelectric TiNiSn can be created and controlled in order to improve physical properties. Ni interstitials in TiNi

_{1.1}

Sn are not thermodynamically stable and, instead, are kinetically trapped using appropriate heat treatments. The Ni interstitials, which act as point defect phonon scattering centers and modify the electronic states near the Fermi level, result in reduced thermal conductivity and enhance the Seebeck coefficient. The best materials tested here, created from controlled heat treatments of TiNi

_{1.1}

Sn samples, display

z T

= 0.26 at 300 K, the largest value reported for compounds in the Ti–Ni–Sn family.

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