# Effect of hydrostatic strain on the mechanical properties and topological phase transition of bi-alkali pnictogen NaLi$_{2}$Bi

## TL;DR

This study investigates how hydrostatic strain affects the mechanical stability and topological phase transitions of NaLi₂Bi, revealing pressure-induced changes in its topological state and auxetic properties relevant for nanoscale device design.

## Contribution

It provides the first detailed analysis of the mechanical properties and topological phase transitions of NaLi₂Bi under hydrostatic pressure using density functional theory.

## Key findings

- NaLi₂Bi is mechanically stable under hydrostatic pressure.
- Hydrostatic pressure induces a topological phase transition in NaLi₂Bi.
- NaLi₂Bi exhibits auxetic behavior with a negative Poisson's ratio.

## Abstract

The bi-alkali pnictogens have attracted significant attention for optoelectronic and photocathodic device applications. However, in most of the compounds belonging to this family, there has been less effort put into investigating the mechanical properties and topological phase transitions (TPT) of the compounds. Here, in the framework of density functional theory, the mechanical properties and topological phase transition of NaLi$_{2}$Bi under hydrostatic pressures are investigated. Elastic constants and phonon calculations have shown the mechanical and dynamical stability of this compound under hydrostatic tension and compression. The analysis of the elastic constants shows that the NaLi$_{2}$Bi in the equilibrium state is an auxetic material with a negative Poisson's ratio of -0.285, which changes to a material with a positive Poisson's ratio under hydrostatic tension. Meanwhile, Poisson's ratio and Pugh ratio indicate that this compound has brittle behavior and maintains it under hydrostatic pressures. The calculated results of the band structure within the generalized gradient approximation (GGA) (Tran-Blaha modified Becke-Johnson exchange potential approximation (TB-mBJ)) show that NaLi$_{2}$Bi is a nontrivial topological material (trivial topological material). It was found that hydrostatic compression (tension) in the GGA (TB-mBJ) approach leads to a transition from a nontrivial (trivial) to a trivial (nontrivial) topological phase for this compound. Moreover, the calculated Wannier charge centers confirm the TPT. Identifying the mechanisms controlling the auxetic behavior and TPT of this compound offers a valuable feature for designing and developing high-performance nanoscale electromechanical and spintronic devices.