Superconductivity up to 243 K in the yttrium-hydrogen system under high pressure

High-temperature superconductivity (HTSC) is technologically important but has long been limited in conventional materials (e.g., MgB₂ at 39 K). The discovery of superconductivity at 203 K in H₃S at ~150 GPa provided a pathway to room-temperature superconductivity via hydrogen-rich, phonon-mediated compounds under pressure. Subsequent work reported Tc ≈ 250–260 K in LaH₁₀ and up to 287 K in a ternary C–S–H system. Yttrium hydrides have been highlighted by theory as particularly promising, with several predicted high-Tc phases and specific crystal structures under pressure. However, discrepancies remain between predicted Tcs/phase stabilities and experimental observations. This study aims to experimentally map phases in the Y–H system at very high pressures, determine their crystal structures, and measure superconducting transport (including isotope effects and magnetic-field dependence) to test theoretical predictions, resolve phase diagram contradictions, and identify the highest-Tc yttrium hydrides achievable under static compression.

Prior breakthroughs include Tc ≈ 203 K in H₃S (~150 GPa) and ≈ 250–260 K in LaH₁₀ (~170 GPa). Multiple groups confirmed high-Tc in H₃S (Tc ~183–200 K) and refined LaH₁₀ near 250 K. Theoretical work predicts several high-Tc yttrium hydrides: fcc YH₃ with Tc potentially 205–326 K at ~250 GPa or ~303 K at 400 GPa; hcp/other YH₆ variants predicted stable near 200 GPa with Tc ~253–276 K; and additional predicted phases (e.g., bcc YH₅) at varying pressures. Reported experimental data on Y hydrides have sometimes conflicted with predictions regarding phase stability ranges and Tc, motivating comprehensive, high-pressure experimental studies to benchmark and refine computational models.

Samples were synthesized and characterized in diamond anvil cells (DACs). A total of 31 samples were prepared by compressing: (i) elemental yttrium in H₂ or D₂; (ii) preformed yttrium trihydride YH₃/D₃; or (iii) mixtures including NH₃BH₃ as a hydrogen source. Reactions were conducted over wide pressure–temperature ranges: Y reacted with H₂ at room temperature from at least 17 GPa; higher hydrogen stoichiometries (YH₄, YH₆, YH₈) formed after laser heating (~1500–2300 K) at ~160–175 GPa and above, with phases observed up to 395 GPa; additional synthesis/annealing at 200–244 GPa (often at cryogenic temperature for extended periods) promoted phase evolution. Pressures up to 410 GPa and temperatures up to ~2250 K were achieved for in situ XRD at high temperature. Crystal structures and phase compositions were determined by synchrotron X-ray powder diffraction. Lattice parameters were measured across 135–410 GPa; equations of state were fitted to extract volumes. Hydrogen stoichiometry was cross-checked via hydrogen-induced volume expansion estimates (VH). XRD experiments used micro-focused beams (spot ~2.5×2.5 µm²), with wavelengths ~0.2532 Å and ~0.1334 Å at synchrotron beamlines and a Pilatus detector. Data reduction used DIOPTAS, with structure refinements performed using GSAS/EXPGUI-type workflows. Pressure determination employed multiple standards depending on sample: diamond anvil Raman edge, ruby luminescence calibration, and in some cases equation of state of Mo. Electrical transport was measured using a four-probe van der Pauw configuration with micro-fabricated leads in the DAC. Magneto-transport up to 9 T was used to suppress superconductivity and estimate upper critical fields. Isotope effects were probed by substituting D for H (YDₓ). Phase fields explored included bct YH₄/YH₆ and bcc YH₅/YH₈ in the ~160–250+ GPa range. The presence of lower hydrides (e.g., fcc/bcc YH₃) near electrical leads—intentionally kept cooler to protect contacts—was monitored by XRD and considered in interpreting transport data.

- Synthesized and identified multiple yttrium hydrides (YH₃, YH₄, YH₆, YH₈) under extreme pressures; determined their crystal structures and pressure–volume relations, consistent with theoretical predictions for lattice parameters and c/a trends. - Observed superconductivity in Im-3m YH₆ and P6₃/mmc YH₈ with maximum Tcs of ~220 K at 183 GPa (YH₆) and ~243 K at 201 GPa (YH₈). Tc exhibits a dome-like pressure dependence, decreasing at higher pressures. - Did not observe the predicted Fm-3m YH₁₀ (with theoretical Tc > 300 K). Instead, YH₈ was the hydrogen-richest yttrium hydride realized within 410 GPa and 2250 K. - Isotope effect: Replacing H with D shifted Tc down to ~168–172 K at 173–205 GPa in corresponding deuterides. Derived isotope effect coefficients α ≈ 0.39 (for one phase) and ≈ 0.50 (for another), indicating a conventional phonon-mediated pairing with possible anharmonic contributions. - Magneto-transport: Applied fields up to 9 T suppressed superconductivity, enabling extrapolation of the upper critical field Hc₂(0). Fits using GL and WHH models gave, for a deuteride phase, Hc₂(0) ≈ 107 T (GL) and ≈ 157 T (WHH), implying coherence lengths ξ(0) ≈ 1.45–1.75 nm and 2.3–2.7 nm depending on sample/model. For the hydride counterpart, estimated Hc₂(0) ≈ 92–120 T. Higher Tc generally correlates with higher Hc₂ across superhydrides, consistent with prior systems (e.g., LaH₁₀, H₃S, ThH₁₀/ThH₆, CeH₆). - Structural and volumetric data support phase assignments; hydrogen-induced volume expansion values (VH ~1.7–1.8 ų/H atom at 180–200 GPa for bcc phases) align with other metal hydrides, aiding stoichiometry estimation.

The experiments validate key theoretical predictions regarding crystal structures and overall compressibility behavior of yttrium superhydrides under pressure, while revealing systematic Tc values lower by ~30 K than many calculations. The observed dome-like Tc(P) trends resemble those in H₃S and LaH₁₀ and may reflect pressure-induced phonon stiffening and electronic structure nuances (e.g., possible multigap behavior). The clear isotope effect (α ≈ 0.39–0.50) confirms a conventional, phonon-mediated mechanism with likely anharmonic contributions. Importantly, the predicted high-Tc fcc YH₁₀ phase was not stabilized despite reaching 410 GPa and high temperatures, suggesting kinetic or thermodynamic barriers under the explored conditions; instead, YH₈ appears to be the hydrogen-rich limit within the studied P–T window. The results address discrepancies between prior theoretical and experimental phase diagrams/Tcs for Y–H, providing benchmarks to refine ab initio predictions and to guide synthesis pathways. Differences with some reports (e.g., higher claimed Tcs and contrasting pressure dependences) likely arise from variations in phase purity, pressure calibration, and synthesis pathways; the present comprehensive structural and transport evidence supports the identified phase–Tc assignments.

This work demonstrates superconductivity up to 243 K at 201 GPa in yttrium superhydrides, identifying Im-3m YH₆ (Tc ~220 K at 183 GPa) and P6₃/mmc YH₈ (Tc ~243 K at 201 GPa) as the leading phases within the accessible P–T range. Comprehensive XRD, transport, isotope, and magneto-transport measurements map the Y–H phase space and confirm phonon-mediated pairing. Despite theoretical predictions, the Fm-3m YH₁₀ phase was not observed; YH₈ is the hydrogen-rich limit reached up to 410 GPa and 2250 K. The study provides critical experimental benchmarks for refining computational models and clarifying phase stability. Future work should pursue alternative synthesis routes to access YH₁₀ or other hydrogen-richer phases, perform higher-field magneto-transport to directly probe Hc₂, reduce temperature/pressure gradients during laser heating to improve phase purity, and explore detailed electron–phonon coupling and anharmonic effects to reconcile theory–experiment Tc gaps.

- The highest predicted phase (Fm-3m YH₁₀) was not realized, limiting direct tests of room-temperature Tc predictions. - Electrical contacts required deliberate underheating near leads, leaving minor lower-hydride impurities in some samples. - Magnetic fields were limited to 9 T, requiring extrapolation (GL/WHH) to estimate Hc₂(0). - Extreme pressure–temperature conditions can introduce gradients and small sample volumes, impacting homogeneity and precise Tc determination. - Pressure calibrations at multi-megabar conditions carry inherent uncertainties depending on the standard (diamond Raman edge, ruby, Mo EoS).