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

The pursuit of room-temperature superconductivity has been a major focus in materials science. While conventional theories seemed to limit Tc, the discovery of unconventional superconductors like cuprates and the breakthrough achievement of 203 K superconductivity in H3S at ~150 GPa, spurred extensive research into hydrogen-rich materials. The yttrium-hydrogen system emerged as a promising candidate due to theoretical predictions of high Tc values. However, inconsistencies existed between predicted and experimentally observed phase diagrams and Tc values. This research aims to experimentally investigate the yttrium-hydrogen system under high pressure, aiming to resolve the existing theoretical and experimental discrepancies and further explore the potential for high-temperature superconductivity in this system. The importance lies in the potential technological advancements enabled by room-temperature superconductors, including energy efficient power transmission and more powerful electronics. The study focuses on resolving the contradictions between theory and experiment, refining predictive models for high-Tc materials, and potentially contributing to the development of room-temperature superconductors. This is a critical step in understanding the behavior of hydrogen-rich compounds under extreme conditions and to push the boundaries of known superconductivity.

Previous research significantly advanced the field of high-temperature superconductivity (HTSC). The discovery of superconductivity in H3S at 203 K under pressure marked a significant step toward room-temperature superconductivity. Subsequent studies focused on other hydrogen-rich compounds, with LaH10 achieving Tc of 252 K at 170 GPa and ternary systems showing even higher Tc values. The yttrium-hydrogen system was theoretically predicted to exhibit remarkably high Tc values, potentially exceeding 300 K. However, significant discrepancies arose between theory and experiment in terms of the observed phases and transition temperatures. Theoretical studies predicted high Tc for various yttrium hydride phases such as fcc YH3, hexagonal YH6 and bcc YH5, but experimental confirmation and verification were lacking and often contradictory. This paper aims to address these gaps and provide a comprehensive experimental study.

The study employed diamond anvil cells (DACs) to synthesize yttrium hydrides under high pressure. Various yttrium hydrides were synthesized in situ by reacting yttrium metal with hydrogen or deuterium gas under high pressure, and also using NH3BH3 as an alternative source of hydrogen. The synthesis process involved compression of elemental metal in H2/D2 and subsequent heating to facilitate the reaction. Samples were prepared under pressure using a combination of different initial materials and synthesis approaches. A total of 31 samples were synthesized and analyzed. The crystal structures and compositions of the synthesized hydrides were determined by X-ray powder diffraction (XRD), using methods like Rietveld refinement to analyze the patterns and determine the lattice parameters and phase compositions. The compositions were also estimated by analyzing the hydrogen-induced volume expansion. Electrical resistance measurements using a four-probe technique and magneto-transport measurements up to 9 T were performed to identify superconducting transitions and determine the critical temperature (Tc) as a function of pressure and magnetic field. The pressure in the DAC was estimated using the ruby luminescence method and the equation of state of molybdenum. The isotopic effect was investigated by replacing hydrogen with deuterium in the synthesis. The high-pressure experiments conducted utilized a combination of laser heating and cryogenic cooling to access a wide range of thermodynamic conditions. Advanced characterization techniques, such as synchrotron X-ray diffraction, were utilized to precisely determine crystal structures and phase compositions. Sophisticated data analysis techniques were utilized to extract key parameters such as critical temperature, upper critical magnetic field, and isotopic coefficients.

The study successfully synthesized yttrium hydrides with compositions YH3, YH4, YH6, and YH8 under high pressure. Superconductivity was confirmed in the Im-3m YH6 and P63/mmc YH8 phases. The maximum Tc observed was approximately 243 K at 201 GPa for the P63/mmc YH8 phase and 220 K at 183 GPa for the Im-3m YH6 phase. Both phases exhibited a dome-shaped Tc(P) dependence, with Tc decreasing at higher pressures. The isotopic effect was clearly demonstrated by a decrease in Tc when hydrogen was replaced by deuterium, supporting the conventional phonon-mediated mechanism of superconductivity. The isotope effect coefficient α was estimated to be 0.39 for bcc-YH3 and ~0.50 for hcp-YH3, indicating anharmonic effects and acoustic phonon contribution. The upper critical field Hc2 was determined through magneto-transport measurements, showing a significant reduction in Tc with increasing magnetic field, further confirming the superconductivity. The estimated coherence length ξ(0) was approximately 1.45-1.75 nm for hcp-YD and 2.3-2.7 nm for hcp-YH3. Despite a difference in Hc2(0) values for YD3 and YH3, the Fermi velocities were found to be quite similar, indicating that the charge carriers contributing to superconductivity are not significantly different between these phases. The experimental findings revealed a significant difference between the experimentally measured Tc values and the theoretically predicted values for both YH6 and YH8, highlighting the need for refined computational methods. The high-pressure, high-temperature synthesis of YH6 yielded results that confirmed that the theoretical models require adjustments to align with experimental observations. The synthesis conditions reached record pressures and temperatures (up to 410 GPa and 2250 K), which contributed significantly to the success of the study in creating new materials and establishing new benchmarks for high-pressure and high-temperature synthesis.

The observed superconductivity in YH6 and YH8 phases with Tc values exceeding 200 K under high pressure demonstrates the potential of the yttrium-hydrogen system for high-temperature superconductivity. The significant difference between experimental and theoretical Tc values highlights limitations in current computational methods for predicting the superconducting properties of complex hydrides under extreme pressure. The observed isotope effect and magnetic field dependence of Tc strongly support a conventional phonon-mediated mechanism. The close values of Fermi velocities observed in the YH3 and YD3 phases suggest a similarity in the charge carrier behavior. These findings contribute significantly to the ongoing efforts to achieve room-temperature superconductivity and to advance the understanding of the interplay between pressure, hydrogen content, and the emergence of superconductivity in complex hydrides. Future research should focus on improving theoretical models to accurately predict the superconducting properties of such materials and to explore other high-pressure yttrium hydrides or related systems.

This research demonstrated superconductivity in yttrium hydrides at record temperatures, exceeding 240 K under high pressure. The study highlights discrepancies between theoretical predictions and experimental findings, emphasizing the need for further refinements in computational models. The observed isotope effect and magnetic field dependence confirm a conventional phonon-mediated mechanism. The results represent significant progress in the quest for room-temperature superconductivity and underscore the importance of high-pressure synthesis in discovering novel materials with exceptional properties. Future work could focus on exploring ternary or quaternary yttrium-based hydrides under high pressure, with the aim of further increasing Tc and potentially achieving room temperature superconductivity.

The high-pressure experimental conditions inherent in this study restrict scalability and wider application. The synthesis method might have limitations in terms of sample size and homogeneity, potentially affecting the reliability of measurements. The pressure determination method might have inaccuracies that affect the Tc measurements at the highest pressures. Furthermore, the complexity of the phase diagrams and the limited experimental window within the pressure-temperature space might restrict a complete understanding of the phenomena reported in this study.