  We coupled first principles calculations and the quasiharmonic approximation
combined with experiments (X-Ray diffraction and dilatometry measurements) to
determine the thermal properties of NiTiSn (half-Heusler) and Ni2TiSn (Heusler)
compounds. These properties are important especially if they are to be used in
thermoelectric applications. First, the calculation of their mode Gruneisen
parameter shows that it is positive throughout the first Brillouin zone. This
suggests that these compounds undergo a regular thermal expansion. Then, the
calculation of the Ni2TiSn thermal expansion shows an excellent agreement, even
in the high temperature range, with our high energy powder X-Ray diffraction
measurements (ESRF) and dilatometry experiments. In the case of NiTiSn, this
agreement is less impressive. This could be due to stronger phonon-phonon
interactions that are not considered within the quasiharmonic approximation,
but also to the difficulty of making high-quality NiTiSn samples. Finally, the
constant-pressure and constant-volume heat capacities have been calculated for
both compounds and compared with the experimental data reported in the
literature. In particular, we have decomposed the constant-volume heat capacity
of Ni2TiSn into a purely electronic and a phonon-mediated contribution, and we
discuss each of them.
