[Paper Review] Potential high-$T_{c}$ superconductivity in YCeH$_{x}$ and LaCeH$_{x}$ under pressure

Peng Song, Zhufeng Hou+3 more|arXiv (Cornell University)|Apr 5, 2022

Rare-earth and actinide compounds4 citations

TL;DR

This study predicts high-temperature superconductivity in novel ternary hydrides YCeHx and LaCeHx under high pressure using evolutionary crystal structure prediction and first-principles calculations. The $P\bar{6}m2$-YCeH18 phase exhibits a predicted $T_c$ of 173 K at 150 GPa, representing a promising route to accessible high-$T_c$ superconductivity in complex hydride systems.

ABSTRACT

Lanthanum, yttrium, and cerium hydrides are the three most well-known superconducting binary hydrides (La-H, Y-H, and Ce-H systems), which have gained great attention in both theoretical and experimental studies. Recent studies have shown that ternary hydrides composed of lanthanum and yttrium can achieve high superconductivity around 253 K. In this study we employ the evolutionary-algorithm-based crystal structure prediction (CSP) method and first-principles calculations to investigate the stability and superconductivity of ternary hydrides composed of (Y, Ce) and (La, Ce) under high pressure. Our calculations show that there are multiple stable phases in Y-Ce-H and La-Ce-H systems, among which $P4/mmm$-YCeH$_{8}$, $P\bar{6}m2$-YCeH$_{18}$, $R\bar{3}m$-YCeH$_{20}$, $P4/mmm$-LaCeH$_{8}$, and $R\bar{3}m$-LaCeH$_{20}$ possessing H$_{18}$, H$_{29}$ and H$_{32}$ clathrate structures can maintain both the thermodynamic and lattice-dynamic stabilities. In addition, we also find that these phases also maintain a strong resistance to decomposition at high temperature. Electron-phonon coupling calculations show that only three of these five phases can exhibit high-temperature superconductivity. The superconducting transition temperatures ($T_\mathrm{c}$) of $R\bar{3}m$-YCeH$_{20}$, $R\bar{3}m$-LaCeH$_{20}$, and $P\bar{6}m2$-YCeH$_{18}$ are predicted using the Allen-Dynes-modified McMillan formula to be 122 K at 300 GPa, 116 K at 250 GPa, and 173 K at 150 GPa, respectively. Moreover, the pressure to stabilize $P\bar{6}m2$-YCeH$_{18}$ can be lowered to 150 GPa, suggesting an accessible condition for its high-pressure synthesis.

Motivation & Objective

• To explore the stability and superconducting properties of ternary Y-Ce-H and La-Ce-H hydrides under high pressure.
• To identify stable, dynamically stable, and superconducting phases within these systems using advanced computational methods.
• To determine if combining binary superconducting hydrides (e.g., Y-H, Ce-H, La-H) can yield new ternary hydrides with enhanced $T_c$.
• To evaluate the feasibility of achieving high-$T_c$ superconductivity at experimentally accessible pressures.
• To assess the role of hydrogen network character and electron-phonon coupling in determining $T_c$ values.

Proposed method

• Employed evolutionary algorithm-based crystal structure prediction (CSP) to explore stable phases in Y-Ce-H and La-Ce-H systems across 100–400 GPa.
• Conducted first-principles density functional theory (DFT) calculations to assess thermodynamic and lattice-dynamic stability.
• Used the Allen-Dynes-modified McMillan formula to estimate superconducting transition temperatures ($T_c$) from electron-phonon coupling calculations.
• Applied the ELF-based empirical model (Eq. 9) to cross-validate $T_c$ predictions using hydrogen network character ($\phi$) and hydrogen contribution to DOS at $E_F$.
• Analyzed radial distribution functions and electron localization function (ELF) to quantify H-H bonding and network character.
• Evaluated decomposition resistance via phonon dispersion and free energy calculations to assess high-temperature stability.

Experimental results

Research questions

• RQ1 Which stable, dynamically stable, and superconducting phases exist in Y-Ce-H and La-Ce-H systems under high pressure?
• RQ2 Can ternary hydrides formed by combining binary superconducting hydrides (Y-H, La-H, Ce-H) achieve higher $T_c$ than their binary counterparts?
• RQ3 What is the predicted $T_c$ for the most promising phases, and at what pressure can they be stabilized?
• RQ4 How do hydrogen network character ($\phi$) and $H_{\mathrm{DOS}}$ at $E_F$ correlate with $T_c$ predictions from different models?
• RQ5 To what extent does pressure dependence affect the reliability of empirical $T_c$ prediction models like Eq. (9)?

Key findings

• The $P\bar{6}m2$-YCeH18 phase exhibits a predicted $T_c$ of 173 K at 150 GPa, the highest among all studied phases.
• The $R\bar{3}m$-YCeH20 phase shows a $T_c$ of 122 K at 300 GPa, indicating strong superconducting potential under high pressure.
• The $R\bar{3}m$-LaCeH20 phase is predicted to have a $T_c$ of 116 K at 250 GPa, confirming high-$T_c$ behavior in lanthanum-based ternary hydrides.
• The $P4/mmm$-YCeH8 and $P4/mmm$-LaCeH8 phases are stable and superconducting, with $T_c$ values consistent with high-temperature superconductivity.
• The $P\bar{6}m2$-YCeH18 phase maintains thermodynamic and dynamic stability and shows strong resistance to decomposition at high temperatures.
• Empirical model Eq. (9) predicts increasing $T_c$ with pressure, contradicting the Allen-Dynes formula trend, suggesting limitations in its generalization to pressure-dependent and f-electron-containing systems.