On the dynamical stability of ferromagnetic Ru and Os in the bct structure: a first-principles study

TL;DR

This study uses first-principles calculations to analyze the phonon dispersion of ferromagnetic Ru and Os in the bct structure, revealing their dynamical instability and challenging their potential as metastable phases.

Contribution

It provides the first phonon dispersion analysis for ferromagnetic Ru and Os in the bct structure, demonstrating their dynamical instability.

Findings

Both Ru and Os in bct structure show imaginary phonon frequencies.

The structures are dynamically unstable, not metastable.

Results challenge previous predictions of magnetic states in these phases.

Abstract

Recent theoretical studies have predicted magnetic states for Ru and Os in the body centered tetragonal structure (bct) with c/a < 1. In this study, we present first principles calculations of the phonon dispersion for ferromagnetic Ru- and Os-bct along the epitaxial and uniaxial Bain paths, to evaluate their dynamical stability. The phonon dispersions were computed using the density function perturbation theory, including the gradient corrections to the exchange-correlation functional within the plane-waves ultrasoft-pseudopotential approximation. The phonon dispersion for the local minimum in the Bain path with c/a < 1 as well as the uniaxial and epitaxial strained structures are analyzed. We find imaginary frequencies along different directions of the Brillouin zone, which indicates that both systems are dynamically unstable. Consequently, ferromagnetic Ru and Os in the body centered tetragonal structure with c/a < 1 are not truly metastable phases.