[Paper Review] Superconductivity up to 243 K in yttrium hydrides under high pressure

P. P. Kong, Vasily S. Minkov+10 more|arXiv (Cornell University)|Sep 23, 2019

High-pressure geophysics and materials29 references78 citations

TL;DR

The paper reports synthesis of YH4, YH6, and YH9 under pressures up to 237 GPa and observes superconductivity with Tc up to 243 K for YH9 at 201 GPa, with YH6 reaching 227 K at 237 GPa, while YH10 was not stabilized.

ABSTRACT

The discovery of high-temperature conventional superconductivity in H3S with a critical temperature of Tc=203 K was followed by the recent record of Tc ~250 K in the face-centered cubic (fcc) lanthanum hydride LaH10 compound. It was realized in a new class of hydrogen-dominated compounds having a clathrate-like crystal structure in which hydrogen atoms form a 3D framework and surround a host atom of rare earth elements. Yttrium hydrides are predicted to have even higher Tc exceeding room temperature. In this paper, we synthesized and refined the crystal structure of new hydrides: YH4, YH6, and YH9 at pressures up to 237 GPa finding that YH4 crystalizes in the I4/mmm lattice, YH6 in Im-3m lattice and YH9 in P63/mmc lattice in excellent agreement with the calculations. The observed very high-temperature superconductivity is comparable to that found in fcc-LaH10: the pressure dependence of Tc for YH9 also displays a "dome like shape" with the highest Tc of 243 K at 201 GPa. We also observed a Tc of 227 K at 237 GPa for the YH6 phase. However, the measured Tcs are notably lower by ~30 K than predicted. Evidence for superconductivity includes the observation of zero electrical resistance, a decrease of Tc under an external magnetic field and an isotope effect. The theoretically predicted fcc YH10 with the promising highest Tc>300 K was not stabilized in our experiments under pressures up to 237 GPa.

Motivation & Objective

• Motivate search for hydrogen-dominated superconductors with very high Tc under extreme pressures.
• Refine crystal structures of YH4, YH6, YH9 and map their Tc as a function of pressure.
• Compare experimental Tc and structure with theoretical predictions for hydrogen-rich hydrides.

Proposed method

• Synthesize yttrium hydrides (YH4, YH6, YH9) at pressures up to 237 GPa.
• Determine crystal structures: YH4 (I4/mmm), YH6 (Im-3m), YH9 (P63/mmc).
• Measure superconductivity via zero resistance, Tc suppression under magnetic field, and isotope effect.
• Compare experimental Tc with theoretical predictions for H-dominated clathrate-like hydrides.

Experimental results

Research questions

• RQ1 What is the highest Tc achievable in yttrium hydrides under high pressure?
• RQ2 How do crystal structures of YH4, YH6, and YH9 relate to their superconducting properties?
• RQ3 Does the experimental Tc align with theoretical predictions for fcc YH10 and related hydrides under high pressure?

Key findings

• YH4 crystallizes in I4/mmm, YH6 in Im-3m, and YH9 in P63/mmc under high pressure up to 237 GPa.
• Tc reaches 243 K for YH9 at 201 GPa (dome-shaped Tc vs pressure).
• Tc for YH6 reaches 227 K at 237 GPa.
• Measured Tcs are about 30 K lower than some theoretical predictions.
• Evidence for superconductivity includes zero resistance, Tc suppression by magnetic field, and isotope effect.
• The predicted fcc YH10 with Tc > 300 K was not stabilized up to 237 GPa.