Thermal Expansion of Ni-Ti-Sn Heusler and Half-Heusler Materials from First-Principles Calculations and Experiments

We coupled first-principles calculations and 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 of their mode Gruneisen parameter shows that is it 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.