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

TL;DR

This study combines first principles calculations with experimental methods to analyze the thermal expansion and heat capacities of NiTiSn and Ni2TiSn compounds, relevant for thermoelectric applications, highlighting good agreement for Ni2TiSn.

Contribution

It provides a combined computational and experimental analysis of thermal properties of NiTiSn and Ni2TiSn, with detailed insights into their thermal expansion and heat capacities, including electronic and phonon contributions.

Findings

Ni2TiSn shows excellent agreement between calculations and experiments.

NiTiSn's thermal expansion agreement is less precise, possibly due to phonon interactions.

Heat capacities are decomposed into electronic and phonon parts for Ni2TiSn.

Abstract

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.