High-temperature shape memory loss in nitinol: a first principles study

First-principles calculations to investigate physical properties of orthorhombic perovskite YBO3 (B = Ti & Fe) for high energy applications

Orthorhombic oxide perovskite compounds are very promising materials for the applications of optoelectronics and thermal barrier coating. This work represents a numerical simulation of YBO₃ compounds through the first-principles ab initio approach. The electronic and magnetic properties are investigated employing the general gradient approximation (GGA) coupled to the integration of the Hubbard U-term which is the GGA + U. The Ti and Fe-based YBO₃ perovskite compounds are found to be actively promising within the ferromagnetic configuration and their lattice parameters are consistent with the previous studies. The calculations of formation energy signify that the compounds YBO₃ are stable thermodynamically. The electronic properties are computed and evaluated by the band structure and density of states for both compounds and the results depict that these materials are ferromagnetic half-metallic. Mechanically these compounds are stable, ductile, anisotropic, and hard to scratch. The thermal properties are evaluated for YBO₃ (B = Ti and Fe) compounds up to a temperature range of 2000 K. This work can open new opportunities for further exploration in this field.

First-principles determination of the crystallography of the low-temperature phase for NbRu and TaRu shape memory alloys

The crystallography of the low-temperature phases (β'') for shape memory alloys (NbRu and TaRu) has been debated for decades. Though a P2/m monoclinic structure has been proposed for the β'' phase, the proposed structure is not able to completely represent the measured diffraction data. In this work, the crystallography of the β'' phase was investigated by first-principles calculations. We showed that the previously reported P2/m monoclinic structure was lattice unstable due to the presence of the soft phonon mode. A P2₁/m monoclinic structure was derived from the P2/m monoclinic structure by displacing its atoms according to the eigenvector of the soft phonon mode at the Γ point. The P2₁/m and the P2/m monoclinic structures are structurally similar, but the former one is energetically and structurally more favorable than the latter one. We concluded that the β'' phase preferred to crystallize in the P2₁/m monoclinic structure rather than the previously reported P2/m monoclinic structure. Our results offer guidance for the experimental determination of the crystallography of the β'' phase for NbRu and TaRu.

Properties of Alkaline-Earth-Metal Polynitrogen Ternary Materials at High Pressure

We report the formation of cubic and tetragonal BaSrN3 at 100 GPa using an ab initio structure search method. Pressure ramping to 0 GPa reveals a reaction pressure threshold of 4.92 and 7.23 GPa for the cubic and tetragonal BaSrN3, respectively. The cubic phase is stabilized by coulombic interaction between the ions. Meanwhile, tetragonal BaSrN3 is stabilized through an expansion of the d-orbital in Ba and Sr atoms that is compensated by delocalization of π-electrons in N through reduction of π overlap. Elastic properties analysis suggests that both phases are mechanically stable. The structures also have high melting points as predicted using an empirical model, and all imaginary modes vanishes at about 2000 K. These results have significant implication for the design of cleaner and environmentally friendly high energy density materials.