Revisiting Phase Transitions of Yttrium: Insights from Density Functional Theory

TL;DR

This study uses density functional theory to analyze yttrium's phase transitions, revealing the importance of vibrational instabilities and soft phonon modes in driving structural changes, with accurate predictions from the r2SCAN functional.

Contribution

It provides a systematic DFT analysis of yttrium's phase transitions, highlighting the role of vibrational instabilities and validating the r2SCAN functional for accurate transition pressure predictions.

Findings

r2SCAN functional accurately predicts phase transition pressures

Vibrational instabilities drive phase transitions in yttrium

Soft acoustic phonon modes emerge at phase boundaries

Abstract

Understanding the mechanism of structural phase transitions in rare-earth elements is a fundamental challenge in condensed matter physics, with significant implications for materials science applications. In this study, we present a systematic investigation on the phase transitions of yttrium under low-pressure conditions ($<$30 GPa) focusing on the hcp, Sm-type, and dhcp phases. A comparative analysis of the generalized gradient approximation (GGA) and meta-GGA functionals reveals that the PBE-GGA functional significantly underestimates the phase transition pressures, whereas the r$^2$SCAN functional provides accurate predictions of phase transition pressures which are in excellent agreement with experimental data. The results confirm that the phase transitions in yttrium are driven by vibrational instabilities, as evidenced by the emergence of soft acoustic modes in the phonon dispersion curves for the hcp and Sm-type phase. Elastic properties calculations further confirm mechanical softening at the phase boundaries, particularly in the hcp phase, suggesting a strong correlation between elastic instability and structural transitions. These findings suggest that the emergence of soft modes in the phonon dispersion curves might be a key factor driving the structural phase transition in the rare earth materials.