Mechanical stability and origin of half-metallicity of new M(2)NiZ (M=Sc, Ti, and V; Z=Tl and Pb) Heusler alloys

A theoretical investigation of electronic and magnetic properties has been performed on the new members of Heusler alloys M(2)NiZ (M=Sc, Ti, and V; Z=Tl and Pb) that crystallize in an inverse Heusler XA structure. The overall electronic properties and magnetic moments are predominated by M atoms, where the total magnetic moment varies linearly with the number of valence electrons, following the Slater-Pauling rule with ferro- or ferri-magnetic ground states. Their Curie temperatures are well above the room temperature and are comparable to analogous compounds. Among the sample alloys, Sc2NiTl, Ti2NiTl, and Ti2NiPb are half-metals, whereas V2NiTl has a nearly half-metallic profile that can be tuned into novel half-metal under uniform strain. Their thermodynamic, mechanical, and dynamical stabilities are also verified from their formation energy, elastic constants, and phonon spectra, respectively. A strong correlation between the directional elastic sound velocities and spatial dependence of elastic moduli is also observed. The evaluated Poisson's ratio from elastic constants and charge density plots predicts the partial ionic nature of Ti2NiTl and Ti2NiPb and the covalent nature of Sc2NiTl and V2NiTl. The optical phonon modes are found to be both Raman and infrared active, whereas the reststrahlen band is observed in the far-infrared region.