[Paper Review] Synthesis and Superconductivity in Yttrium-Cerium Hydrides at Moderate Pressures

Liu-Cheng Chen, T. Luo+8 more|arXiv (Cornell University)|Aug 10, 2022

Rare-earth and actinide compounds4 citations

TL;DR

This study demonstrates the synthesis of Y₀.₅Ce₀.₅H₉ hydride at moderate pressures (114–120 GPa), achieving a superconducting transition temperature (Tc) up to 140 ± 2 K, significantly higher than in cerium hydrides and lower in pressure than required for yttrium hydrides. The alloy combines the high-Tc potential of Y-H with the low-pressure stabilization of Ce-H, achieving a dynamically stable P6₃/mmc phase with theoretical Tc ≈ 119 K, confirming alloying as a viable route to high-Tc superconductivity at accessible pressures.

ABSTRACT

Inspired by the high critical temperature in yttrium superhydride and the low stabilized pressure in superconducting cerium superhydride, we carry out four independent runs to synthesize yttrium-cerium alloy hydrides. The phases examined by the Raman scattering and x-ray diffraction measurements. The superconductivity is detected with the zero-resistance state at the critical temperature in the range of 97-140 K at pressures ranging from 114 GPa to 120$\pm$4 GPa. The maximum critical temperature of the synthesized hydrides is larger than those reported for cerium hydrides, while the corresponding stabilized pressure is much lower than those for superconducting yttrium hydrides. The structural analysis and theoretical calculations suggest that the phase of Y$_{0.5}$Ce$_{0.5}$H$_9$ has the space group $P6_3/mmc$ with the calculated critical temperature of 119 K, in fair agreement with the experiments. These results indicate that alloying superhydrides indeed can maintain relatively high critical temperature at modest pressures accessible by many laboratories.

Motivation & Objective

• To explore high-Tc superconductivity in rare earth hydrides at experimentally accessible pressures.
• To overcome the limitation of extreme pressures required for superconducting yttrium hydrides (Y-H) and low-Tc in cerium hydrides (Ce-H).
• To investigate whether alloying Y and Ce in hydrides can yield high-Tc superconductivity at lower stabilized pressures.
• To experimentally synthesize and characterize Y₀.₅Ce₀.₅H₉ under high pressure and verify its superconducting transition.

Proposed method

• Synthesis of Y₀.₅Ce₀.₅H₉ using laser heating in symmetric diamond anvil cells (DACs) with ammonia borane as a hydrogen source.
• Structural characterization via X-ray diffraction at the Shanghai Synchrotron Radiation Facility using a focused beam (≤2 μm) and Le Bail refinement.
• Raman scattering measurements to identify phonon modes and confirm phase formation, including a 120 cm⁻¹ mode as a signature of P6₃/mmc-Y₀.₅Ce₀.₅H₉.
• Electrical resistance measurements using Pt leads on rhenium gaskets insulated with c-BN/epoxy to detect zero-resistance transitions.
• Density functional theory (DFT) calculations of electronic, phononic, and superconducting properties, including Eliashberg spectral function and electron-phonon coupling (λ).
• Numerical solution of the Eliashberg equation with μ* = 0.1–0.15 to predict Tc, validated against experimental data.

Experimental results

Research questions

• RQ1 Can Y₀.₅Ce₀.₅H₉ be synthesized at moderate pressures to achieve high-Tc superconductivity?
• RQ2 Does the P6₃/mmc phase of Y₀.₅Ce₀.₅H₉ exhibit a dynamically stable structure at pressures below 180 GPa?
• RQ3 What is the contribution of Y, Ce, and H orbitals to the electronic density of states near the Fermi level in Y₀.₅Ce₀.₅H₉?
• RQ4 How does the electron-phonon coupling strength (λ) in Y₀.₅Ce₀.₅H₉ compare to that in binary Y-H and Ce-H compounds?
• RQ5 Can theoretical predictions of Tc in Y₀.₅Ce₀.₅H₉ match experimental observations?

Key findings

• The highest experimentally observed Tc in Y₀.₅Ce₀.₅H₉ is 140 ± 2 K at 120 ± 3 GPa, exceeding the Tc of 115 K in CeH₁₀ and 57 K in CeH₉.
• The P6₃/mmc-Y₀.₅Ce₀.₅H₉ phase is dynamically stable at 180 GPa and exhibits a large density of states near the Fermi level, primarily from H-s, Ce-f, and Y-d orbitals.
• The calculated electron-phonon coupling constant λ is 2.23 at 180 GPa, indicating strong coupling, with high-frequency H-derived phonons dominating the coupling.
• Theoretical Tc values of 104–119 K are obtained by solving the Eliashberg equation with μ* = 0.1–0.15, in fair agreement with the experimental Tc of 119 K.
• Coexistence of multiple phases, including P6₃/mmc-Y₀.₅Ce₀.₅H₉, C2/m-Y₀.₅Ce₀.₅H₇, and I4/mmm-Y₀.₅Ce₀.₅H₄, is confirmed by XRD and Raman data.
• The 120 cm⁻¹ Raman mode is identified as the lowest optical phonon mode and serves as a key experimental signature for the P6₃/mmc phase.