First-principles prediction of phase transition of YCo$_5$ from self-consistent phonon calculations

TL;DR

This study uses first-principles self-consistent phonon calculations to predict the phase transition temperature of YCo$_5$ from hexagonal to orthorhombic structure and explores the magnetic properties of both phases.

Contribution

It provides a theoretical prediction of the orthorhombic-to-hexagonal phase transition temperature and analyzes magnetic properties using advanced phonon and spin-orbit calculations.

Findings

Orthorhombic phase is more stable at low temperatures.

Hexagonal phase stabilized by phonon entropy at high temperatures.

Transition temperature estimated at approximately 165 K.

Abstract

Recent theoretical study has shown that the hexagonal YCo$_5$ is dynamically unstable and distorts into a stable orthorhombic structure. In this study, we show theoretically that the orthorhombic phase is energetically more stable than the hexagonal phase in the low-temperature region, while the phonon entropy stabilizes the hexagonal phase thermodynamically in the high-temperature region. The orthorhombic-to-hexagonal phase transition temperature is $\sim$165 K, which is determined using the self-consistent phonon calculations. We investigate the magnetocrystalline anisotropy energy (MAE) using the self-consistent and non-self-consistent (force theorem) calculations with the spin-orbit interaction (SOI) along with the Hubbard $U$ correction. Then, we find that the orthorhombic phase has similar MAE, orbital moment, and its anisotropy to the hexagonal phase when the self-consistent calculation with the SOI is performed. Since the orthorhombic phase still gives magnetic properties comparable to the experiments, the orthorhombic distortion is potentially realized in the low-temperature region, which awaits experimental exploration.