  ZrNiSn and related half Heusler compounds are candidate materials for
efficient thermoelectric energy conversion with a reported thermoelectric
figure-of-merit of n-type ZrNiSn exceeding unity. Progress on p-type materials
has been more limited, which has been attributed to the presence of an impurity
band, possibly related to the presence of Ni interstitials in nominally vacant
4d position. The specific energetic position of this band, however, has not
been resolved. Here, we report results of a concerted theory-experiment
investigation for a nominally undoped ZrNiSn, based on measurements of
electrical resistivity, Hall coefficient, Seebeck coefficient and Nernst
coefficient, measured in a temperature range from 80 to 420 K. The results are
analyzed with a semi-analytical model combining a density functional theory
(DFT) description for ideal ZrNiSn, with a simple analytical correction for the
impurity band. The model provides a good quantitative agreement with
experiment, describing all salient features in the full temperature span for
the Hall, conductivity, and Seebeck measurements, while also reproducing key
trends in the Nernst results. This comparison pinpoints the impurity band edge
to 40 meV below the conduction band edge, which agrees well with a separate DFT
study of a supercell containing Ni interstitials. Moreover, we corroborate our
result with a separate study of ZrNiSn0.9Pb0.1 sample showing similar agreement
with an impurity band edge shifted to 32 meV below the conduction band.
