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We coupled first principles calculations and the quasiharmonic approximationcombined with experiments (X-Ray diffraction and dilatometry measurements) todetermine the thermal properties of NiTiSn (half-Heusler) and Ni2TiSn (Heusler)compounds. These properties are important especially if they are to be used inthermoelectric applications. First, the calculation of their mode Gruneisenparameter shows that it is positive throughout the first Brillouin zone. Thissuggests that these compounds undergo a regular thermal expansion. Then, thecalculation of the Ni2TiSn thermal expansion shows an excellent agreement, evenin the high temperature range, with our high energy powder X-Ray diffractionmeasurements (ESRF) and dilatometry experiments. In the case of NiTiSn, thisagreement is less impressive. This could be due to stronger phonon-phononinteractions that are not considered within the quasiharmonic approximation,but also to the difficulty of making high-quality NiTiSn samples. Finally, theconstant-pressure and constant-volume heat capacities have been calculated forboth compounds and compared with the experimental data reported in theliterature. In particular, we have decomposed the constant-volume heat capacityof Ni2TiSn into a purely electronic and a phonon-mediated contribution, and wediscuss each of them.

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