First-principles study of the lattice instabilities in Mn$_{2}$Ni{\it X} ({\it X}= Al, Ga, In, Sn) magnetic shape memory alloys

TL;DR

This study uses first-principles DFT calculations to analyze lattice instabilities in Mn2NiX (X=Al, Ga, In, Sn) magnetic shape memory alloys, revealing phonon softening and potential pre-martensitic phases.

Contribution

It provides a detailed first-principles analysis of phonon instabilities and their origins in Mn2NiX alloys, linking vibrational anomalies to electronic structure features.

Findings

Complete softening in acoustic TA2 branches along [ξξ0] directions

Phonon anomalies linked to Fermi surface nesting and optical branch repulsion

Indications of possible complex pre-martensitic phases

Abstract

Using first-principles based Density Functional Theory (DFT), we have investigated the structural instabilities in the austenite phases of Mn$_{2}$Ni{\it X} ({\it X}= Al, Ga, In, Sn) magnetic shape memory alloys (MSMA). A complete softening is observed in the acoustic TA$_{2}$ branches for all the materials along [$\xi\xi$0] directions leading to the instability in the austenite structure which effectively stabilizes into martensitic structure. The reasons behind this softening are traced back to the repulsion from the optical T$_{2g}$ branches and to the nesting features in the Fermi surfaces. The vibrational density of states, the force constants and the elastic moduli are also computed and analyzed, which reconfirm the underlying mechanism behind the instabilities. The results indicate that the phonon anomalies are related to the occurrence of possible pre-martensitic phases which can be quite complex.