Ab initio prediction of stable nanotwin double layers and 4O structure in Ni$_{2}$MnGa

TL;DR

This study uses ab initio calculations to identify a new stable orthorhombic 4O structure in Ni$_{2}$MnGa, formed by nanotwin double layers, which is energetically favored over known martensitic phases.

Contribution

The paper reveals a previously unknown stable 4O structure in Ni$_{2}$MnGa, highlighting the role of nanotwin double layers in martensitic phase stability.

Findings

4O structure has lowest energy among martensitic phases

Total energy is 1.98 meV/atom lower than nonmodulated martensite

Nanotwin double layers are crucial for structural stability

Abstract

The ab initio electronic structure calculations of the Ni$_{2}$MnGa The alloy indicate that the orthorhombic 4O structure exhibits the lowest energy compared to all known martensitic structures. The 4O structure is formed by nanotwin double layers, i.e., oppositely oriented nanotwins consisting of two (101) lattice planes of nonmodulated martensitic structure. It exhibits the lowest occupation of density of states at the Fermi level. The total energy 1.98 meV/atom below the energy of nonmodulated martensite is achieved within structural relaxation by shifting Mn and Ga atoms at the nanotwin boundaries. The same atomic shift can also be found in other martensitic nanotwinned or modulated structures such as 10M and 14M, which indicates the importance of the nanotwin double layer for the stability of these structures. Our discovery shows that the nanotwinning or modulation is a natural property of low-temperature martensitic phases in Ni-Mn-Ga alloys.