Room-Temperature Superconductivity at 298 K in Ternary La-Sc-H System at High-pressure Conditions (2510.01273v1)

Abstract: Room-temperature superconductor has been a century-long dream of humankind. Recent research on hydrogen-based superconductors (e.g., CaH6, LaH10, etc.) at high-pressure conditions lifts the record of superconducting critical temperature (Tc) up to ~250 kelvin. We here report the experimental synthesis of the first-ever room-temperature superconductor by compression on a mixture of La-Sc alloy and ammonia borane at pressures of 250-260 gigapascals (GPa) via a diamond anvil cell by a laser-heating technique. Superconductivity with an onset temperature of 271-298 kelvin at 195-266 GPa is observed by the measurement of zero electrical resistance and the suppression of Tc under applied magnetic fields. Synchrotron X-ray diffraction data unambiguously reveal that this superconductor crystallizes in a hexagonal structure with a stoichiometry LaSc2H24, in excellent agreement with our previous prediction1. Through thirteen reproducible experimental runs, we provide solid evidence of the realization of a room-temperature superconductor for the first time, marking a milestone in the field of superconductivity.

• The paper demonstrates room-temperature superconductivity up to 298 K in LaSc₂H₂₄ at pressures around 260 GPa.
• It employs diamond anvil cell synthesis with pulsed laser heating to form a hexagonal P6/mmm composite hydrogen network.
• The study highlights scandium’s role in stabilizing unique hydrogen cage motifs and increasing the electron density conducive to superconductivity.

Room-Temperature Superconductivity in LaSc₂H₂₄ at Extreme Pressures

Introduction

The synthesis and characterization of LaSc₂H₂₄, a ternary lanthanum-scandium superhydride, exhibiting superconductivity at and above room temperature (up to 298 K) under high-pressure conditions, represents a significant advance in the field of high-$T_c$ superconductivity. This work builds on the paradigm of hydrogen-rich clathrate hydrides, extending the compositional space from binary to ternary systems, and demonstrates the critical role of compositional and structural tuning in achieving unprecedented superconducting transition temperatures.

Experimental Synthesis and Characterization

Synthesis Protocol

LaSc₂H₂₄ was synthesized by compressing a La-Sc alloy (1:2 atomic ratio) with ammonia borane (NH₃BH₃) as a hydrogen source in a diamond anvil cell (DAC), followed by pulsed laser heating at pressures of 250–260 GPa. The synthesis protocol was optimized based on prior theoretical predictions of phase stability and superconducting properties, with the target phase predicted to be thermodynamically stable above 167 GPa.

Structural Determination

Synchrotron X-ray diffraction (XRD) confirmed the formation of a hexagonal P6/mmm phase, with lattice parameters in close agreement with theoretical predictions. The structure features two distinct hydrogen cages: H₂₄ cages surrounding Sc and H₃₀ cages surrounding La, forming a nested, composite hydrogen network not observed in binary hydrides. The refined stoichiometry, LaSc₂H₂₄, was validated by lattice volume analysis, with minor deviations attributed to hydrogen non-stoichiometry and dehydrogenation during decompression.

Electrical and Magnetic Measurements

Four-probe electrical transport measurements revealed abrupt resistance drops to zero at onset temperatures ($T_\text{onset}$) ranging from 271 K to 298 K at pressures between 195 and 266 GPa. The zero-resistance state was reproducibly observed in multiple independent DAC runs, with the highest $T_c$ (298 K) at 260 GPa. The superconducting transition was further corroborated by the suppression of $T_c$ under applied magnetic fields up to 12 T, with upper critical fields $H_{c2}(0)$ extrapolated to 89–228 T depending on the fitting model (Ginzburg-Landau, WHH, or linear). The extracted coherence lengths (1.3–1.9 nm) are consistent with type-II superconductivity.

Mechanism and Role of Scandium

The introduction of Sc into the La-H system is pivotal for the observed enhancement in $T_c$. Scandium, with its lighter atomic mass and smaller radius compared to La, enables the stabilization of unique hydrogen cage motifs and increases the hydrogen-derived density of states at the Fermi level. The absence of localized $f$-electrons in Sc avoids magnetic pair-breaking effects, in contrast to other rare-earth dopants (e.g., Ce, Nd). The resulting structure is not a disordered solid solution but a well-ordered, composite clathrate, which is essential for the observed superconducting properties.

Comparison with Binary and Other Ternary Hydrides

Previous records for $T_c$ in hydride superconductors were held by binary LaH₁₀ (250 K at ~180 GPa) and related systems. Attempts to enhance $T_c$ via random alloying or doping in binary hydrides have yielded only incremental improvements, primarily due to the lack of new structural motifs. In contrast, the LaSc₂H₂₄ phase represents a distinct structural prototype, with the formation of new hydrogen frameworks directly linked to the dramatic increase in $T_c$.

Limitations and Open Questions

• Pressure Requirement: The necessity of pressures above 250 GPa for phase stability and high $T_c$ remains a significant barrier to practical applications. The possibility of stabilizing this or related phases at lower pressures via chemical precompression or alternative compositional tuning is an open research direction.
• Hydrogen Content and Stoichiometry: Precise determination of hydrogen positions and content is limited by current XRD capabilities. Dehydrogenation and non-stoichiometry during decompression introduce sample variability, affecting $T_c$ and phase stability.
• Direct Magnetic Characterization: While suppression of $T_c$ under magnetic field is strong evidence for superconductivity, direct observation of the Meissner effect at these pressures remains technically challenging. Advances in quantum sensing and high-pressure NMR may address this gap.
• Nature of Superconductivity: Isotope substitution (H/D) and tunneling spectroscopy are needed to definitively establish the pairing mechanism and gap symmetry, though the evidence points toward conventional phonon-mediated superconductivity.

Implications and Future Directions

The demonstration of room-temperature superconductivity in a well-characterized, structurally resolved ternary hydride provides a concrete platform for further exploration of multinary superhydrides. The results suggest that targeted compositional and structural engineering, guided by ab initio crystal structure prediction, can yield further increases in $T_c$ and potentially reduce the required synthesis pressures. The unique hydrogen frameworks observed in LaSc₂H₂₄ may serve as templates for the design of new superconductors with tailored properties.

Future research should focus on:

• Extending the compositional space to include other light elements or transition metals.
• Developing synthesis strategies for metastable retention of high-$T_c$ phases at lower pressures.
• Employing advanced characterization techniques (e.g., high-pressure NMR, quantum magnetometry, tunneling spectroscopy) to probe the superconducting state and hydrogen sublattice.
• Systematic isotope effect studies to confirm the conventional nature of the superconductivity.

Conclusion

The synthesis and characterization of LaSc₂H₂₄ establishes the existence of a room-temperature superconductor with a well-defined crystal structure and mechanism, underlining the potential of ternary and higher-order hydrides for achieving even higher $T_c$ values. The work provides a robust experimental and theoretical framework for the rational design of next-generation superconductors, with significant implications for condensed matter physics and materials science.