Phase Transition and Electronic Structures of All--Metal Heusler-Type XMnTi Compounds (X = Pd, Pt, Ag, Au, Cu, and Ni).

In this work, we investigated the phase transition and electronic structures of some newly designed all--metal Heusler compounds, XMnTi (X = Pd, Pt, Ag, Au, Cu, and Ni), by means of the first principles. The competition between the XA and L2 structures of these materials was studied, and we found that XMnTi favors to feature the L2-type structure, which is consistent with the well-known site-preference rule (SPR). Under the L2 structure, we have studied the most stable magnetic state of these materials, and we found that the ferromagnetic state is the most stable due to its lower energy. Through tetragonal deformation, we found that the L2 structure is no longer the most stable structure, and a more stable tetragonal L1 structure appeared. That is, under the tetragonal strain, the material enjoys a tetragonal phase transformation (i.e., from cubic L2 to tetragonal L1 structure). This mechanism of L2-L1 structure transition is discussed in detail based on the calculated density of states. Moreover, we found that the energy difference between the most stable phases of L1 and L2, defined as Δ (Δ = - ), can be adjusted by the uniform strain. Finally, the phonon spectra of all tetragonal XMnTi (X = Pd, Pt, Ag, Au, Cu, and Ni) phases are exhibited, which provides a powerful evidence for the stability of the tetragonal L1 state. We hope that our research can provide a theoretical guidance for future experimental investigations.