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 methods, we predict that AFM $B2$ FeRh becomes unstable at ambient pressure and transforms without a barrier to an AFM(111) orthorhombic (martensitic) ground state below $90\pm 10\mathrm{K}$. We also consider competing structures, in particular, a tetragonal AFM(100) phase that is not the global ground state, as proposed [Phys. Rev. B 94, 180407(R) (2016)], but a constrained solution.

• Received 11 October 2017
• Revised 25 November 2017

DOI:https://doi.org/10.1103/PhysRevB.97.014202