FeRh groundstate and martensitic transformation

TL;DR

This study predicts a low-temperature martensitic transformation in FeRh from its cubic antiferromagnetic phase to an orthorhombic structure, driven by phonon instabilities, using advanced computational methods.

Contribution

It introduces a comprehensive computational analysis predicting a barrierless martensitic transformation in FeRh, revealing a new low-temperature groundstate.

Findings

FeRh becomes unstable at ambient pressure below 90K.

Predicted transformation is barrierless and involves a change to an orthorhombic structure.

Competing tetragonal phase is not the global groundstate.

Abstract

Cubic B2 FeRh exhibits a metamagnetic transition [(111) antiferromagnet (AFM) to ferromagnet (FM)] around 353 K and remains structurally stable at higher temperatures. However, the calculated zero-Kelvin phonons of AFM FeRh exhibit imaginary modes at M-points in the Brillouin zone, indicating a premartensitic instability, which is a precursor to a martensitic transformation at low temperatures. Combining electronic-structure calculations with ab initio molecular dynamics, conjugate gradient relaxation, and the solid-state nudged-elastic band (SSNEB) methods, we predict that AFM B2 FeRh becomes unstable at ambient pressure and transforms without a barrier to an AFM(111) orthorhombic (martensitic) groundstate below 90K. We also consider competing structures, in particular, a tetragonal AFM(100) phase that is not the global groundstate, as proposed [Phys. Rev. B 94, 180407(R) (2016)], but a constrained solution.