ICT2014 Abstracts

First principles calculations of the vibrational, thermodynamic and mechanical properties of the Ni-Ti-Sn Heusler and half-Heusler compounds have been performed (RSC Advances, 3(44) , 2013, 22176-22184). First, we have calculated the Raman and infrared spectra of NiTiSn, providing benchmark theoretical data directly useful for the assignments of its experimental spectra and clarifying the debate reported in the literature on the assignment of itsmodes. Then, we have discussed the significant vibrational density-of-states of Ni₂TiSn at low-frequencies. These states are at the origin of (i) its smaller free energy, (ii) its higher entropy, and (iii) its lower Debye temperature, with respect to NiTiSn. Then, we have reported the mechanical properties of the two compounds. In particular, we have found that the half-Heusler compound has the largest stiffness. Paradoxically, its bulk modulus is also the smallest. This unusual behavior has been related to the Ni-vacancies that weaken the structure under isostatic compression. Both compounds show a ductile behavior.

We have then in a second study, combined first principles calculations and 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 these materials are to be used in thermoelectric applications. First, we have obtained the mode Gruneisen parameter for both compounds along the high symmetry directions of the first Brillouin zone and show that it never takes negative values. Then the thermal expansion has been calculated for Ni₂TiSn up to 1500K and the agreement is excellent with our high energy X-Ray diffraction measurements (ESRF) and dilatometry experiments. In contrast, the agreement is less satisfactory for NiTiSn due to the difficulty of making high-quality samples. Finally we have calculated the heat capacity (at constant volume and at constant pressure) of both compounds and have also obtained a remarkable agreement with existing experimental data. In particular the heat capacity of Ni₂TiSn has been decomposed into a purely electronic contribution and a phonon-mediated response. Both contributions are discussed.