Room-Temperature Superconductivity in LaSc2H24

• The paper reports that LaSc2H24 exhibits room-temperature superconductivity (271–298 K) via high-pressure synthesis (>195 GPa), validated by structural, resistive, and magnetic measurements.
• The experimental synthesis uses a diamond-anvil cell with pulsed laser heating to create a hexagonal clathrate structure, ensuring precise hydrogen stoichiometry and lattice control.
• Enhanced electron–phonon coupling driven by Jahn–Teller distortions and a unified Fermi surface topology underpins the material’s isotropic superconductivity, paving the way for practical hydride superconductors.

Room-temperature superconductivity in LaSc$_2$H$_{24}$ designates the emergence of zero-resistivity and perfect diamagnetism in a lanthanum–scandium polyhydride with stoichiometry LaSc$_2$H$_{24}$ at temperatures $T_{\mathrm{c}} \sim 271$–$298$ K when compressed above 195–266 GPa. This phenomenon, confirmed via structural, resistive, and magnetic measurements, marks the first reproducible realization of a true superconductor at ambient temperature, and is characterized by a distinctive hexagonal clathrate framework where electronic, phononic, and gap properties fundamentally differ from previous high-$T_{\mathrm{c}}$ hydrides (Song et al., 29 Sep 2025, Wang et al., 4 Jan 2026).

1. Experimental Synthesis and Characterization

LaSc$_2$H$_{24}$ is synthesized by high-pressure reactions of a 1:2 La–Sc alloy and ammonia borane within a diamond-anvil cell (DAC), followed by pulsed laser heating. This procedure yields a black, metallic phase above $\sim$195 GPa, sustaining pressures up to 266 GPa. Key experimental steps include:

• Sample assembly: La and Sc are co-melted or co-sputtered to yield a $33$–$36\%$ : $64$–$66\%$ ratio (verified by EDS) and sandwiched between ammonia borane/hydrogen sources and Pt electrodes in an inert-glovebox atmosphere ($<0.01$ ppm O$_2$/H$_2$O).
• Pressure/temperature conditions: Compression is achieved with 30 $\mu$m diamond culets and Re/epoxy–Al$_2$O$_3$ gaskets, followed by double-sided 1.06 $\mu$m YAG laser heating.
• Structural verification: Synchrotron X-ray diffraction confirms a hexagonal P6/mmm structure (lattice parameters $a=4.86(4)$ Å, $c=3.35(6)$ Å at 254 GPa) with distinct La@H$_{30}$ and Sc@H$_{24}$ cages and a refined hydrogen content $n_{\mathrm{H}}=24.3$ at highest pressures. Rietveld refinements show excellent agreement with predictions and negligible systematic error.
• Hydrogen stoichiometry: Assessed by volume increment, the composition remains near LaSc$_2$H$_{24}$ over 194–266 GPa, with minor dehydrogenation upon decompression.

These methods ensure that the synthesized phase is both structurally and compositionally consistent with theoretical predictions (Song et al., 29 Sep 2025).

2. Superconducting Properties: Observation and Metrics

Room-temperature superconductivity is determined via four-probe resistance and field-suppression measurements in multiple DAC cells:

• Critical temperature: $T_{c,\mathrm{onset}}$ values between 271 K and 298 K at 195–266 GPa, with the highest ($T_{c,\mathrm{onset}}=298$ K) observed at 260 GPa. Zero resistance is measured in selected runs.
• Magnetic field response: Application of external fields ($\leq 9$ T) shifts $T_{c}$ downward by $\sim11$ K, confirming superconducting origin.
• Upper critical field: Evaluated by both Ginzburg–Landau and Werthamer–Helfand–Hohenberg approaches, yielding $\mu_0H_{c2}(0)$ values $89$–$156$ T, with coherence lengths $\xi_{\mathrm{GL}}=1.7$–$1.9$ nm.
• Pressure dependence: $T_{c}$ decreases slightly with pressure in some samples, while in others remains robust above 290 K across 195–266 GPa. Below 190–194 GPa, lattice instability and dehydrogenation suppress superconductivity (Song et al., 29 Sep 2025).

These characteristics, especially the reproducible observation of zero resistance and its suppression by magnetic fields, define LaSc$_2$H$_{24}$ as a room-temperature superconductor by standard criteria.

3. Crystal and Electronic Structure

The crystal structure comprises interleaved La-centered H$_{30}$ and Sc-centered H$_{24}$ clathrate cages in a hexagonal P6/mmm lattice, forming a MgB$_2$-like sublattice order. The principal features are:

• Atomic arrangement: La at (0,0,0), Sc at ($\frac{1}{3},\frac{2}{3},\frac{1}{2}$), hydrogens filling 24–30 sites per formula unit, symmetrically distributed.
• Electronic structure: At the Fermi level ($E_F$), two new Sc–H–Sc motifs emerge:
  • $\sigma$-bands along $\Gamma$–$A$ ($3d_{x^2-y^2}$, $3d_{xy}$ via H$_\mathrm{II}$ bridging),
  • $\pi$-bands along $M$–$K$ ($3d_{zx}$, $3d_{zy}$ via H$_\mathrm{II}$),
  • Retained H–H antibonding states, now elongated compared to LaH$_{10}$.
• Density of states (DOS): Projected DOS at $E_F$ for Sc–H bands is $N_{\mathrm{Sc-H}}(0)=2.1$ states/eV·cell (40% of total $N(0)=5.2$), ensuring substantial electronic participation from hydrogen and scandium.

This structure supports both strong electron–phonon interactions and robust metallicity, which are prerequisites for high-$T_\mathrm{c}$ conventional superconductivity (Wang et al., 4 Jan 2026).

4. Microscopic Mechanism of Room-Temperature Superconductivity

The mechanism leading to high $T_c$ in LaSc$_2$H$_{24}$ fundamentally diverges from the two-gap, anisotropic superconductivity of LaH$_{10}$. The salient features are:

• Jahn–Teller effect and phonon softening: Sc $3d$ orbitals, in the trigonal prismatic environment, induce Jahn–Teller distortion, elongating interlayer H–H bonds from $1.11$ Å (LaH$_{10}$) to $1.20$ Å. This leads to:
  • Lowered electron localization function (ELF $\sim0.5$–0.6), signifying bond metallization;
  • Up to 25% enhancement in H–H antibonding occupancy at $E_F$;
  • Pronounced phonon softening at $q_K=(1/3,1/3,0)$, with a frequency reduction $\Delta\omega\sim200$ cm$^{-1}$.
• Electron–phonon coupling (EPC): The mode at $q_K$ contributes $\lambda_{q_K}\approx0.8$ (20% of total $\lambda$). EPC from all states on the single Fermi surface yields $\lambda_{n\mathbf{k}}\in[2,4]$.
• Fermi-surface topology: Unlike LaH$_{10}$, which possesses disconnected La–H and H–H pockets (two-gap scenario), LaSc$_2$H$_{24}$ exhibits a topologically unified Fermi surface supporting isotropic interactions.
• Gap unification: Migdal–Eliashberg solutions show a single, isotropic gap $\Delta_{n\mathbf{k}}\approx60$ meV at 20 K across all $\mathbf{k}$, contrasting the two distinct gaps ($\Delta_1\approx30$ meV, $\Delta_2\approx70$ meV) in LaH$_{10}$.

These phenomena establish an unprecedented connection between local high-EPC H–H states (enabled by Jahn–Teller effect) and widespread MgB$_2$-like Sc–H channels, culminating in robust, isotropic superconductivity well above ambient temperature (Wang et al., 4 Jan 2026).

5. Electron–Phonon Coupling and $T_\mathrm{c}$ Computation

The superconducting critical temperature in LaSc$_2$H$_{24}$ is rationalized within conventional EPC theory:

• Eliashberg–McMillan theory:

  $$\lambda = 2 \int_0^\infty \frac{\alpha^2 F(\omega)}{\omega}d\omega$$

  with $\alpha^2 F(\omega)$ the Eliashberg spectral function.

• Parameters for LaSc$_2$H$_{24}$ (250 GPa):
  • $$\int_0^{\omega_{max}} \alpha^2 F(\omega)/\omega\,d\omega \approx 1.95 \Rightarrow \lambda \approx 3.9$$ (includes anharmonic corrections)
  • Logarithmic phonon frequency: $\omega_{\ln}\approx900$ K ($\sim625$ cm$^{-1}$)
  • Coulomb pseudopotential: $\mu^* \approx 0.10$
• Allen–Dynes $T_{c}$ formula:

  $$T_{c} = \frac{\omega_{\ln}}{1.2} \exp\left\{ -\frac{1.04(1+\lambda)}{\lambda - \mu^* - 0.62\lambda\mu^*} \right\}$$

• Numerical result:

  $$T_c \approx \frac{900}{1.2}\, \exp\left\{ -\frac{1.04\,\times\,4.9}{3.512} \right\} \approx 300\, \text{K}$$

The large $\lambda$ is primarily attributable to Jahn–Teller–induced soft phonons and the MgB$_2$-like band structure, with no gap anisotropy, accounting for the exceptional $T_c$ (Wang et al., 4 Jan 2026).

6. Structural and Mechanistic Comparisons with Related Hydrides

LaSc$_2$H$_{24}$ emerges within a context of hydride superconductors (e.g., LaH$_{10}$, CaH$_6$, YH$_6$), but unique physical mechanisms distinguish it:

Compound	$T_c$ (K)	$P$ (GPa)	EPC $\lambda$	SC Gap Structure	Key Mechanism
LaH$_{10}$	250–260	170–200	$\sim2$-$8$	Two-gap, anisotropic	Disconnected La–H/H–H pockets
LaSc$_2$H$_{24}$	271–298	195–266	$\sim3.9$	Single-gap, isotropic	Jahn–Teller, FS unification, MgB$_2$-like Sc–H
CaH$_6$	$\sim215$	$\sim150$	$\sim2.7$	Single-gap	Clathrate $H_{24}$ cages

LaSc$_2$H$_{24}$ uniquely integrates local bond softening (Jahn–Teller) and extended band structure topology, facilitating a uniform gap and raising $T_c$ above those of all previously confirmed superconducting hydrides (Song et al., 29 Sep 2025, Wang et al., 4 Jan 2026).

7. Implications and Outlook

The successful synthesis and mechanistic elucidation of LaSc$_2$H$_{24}$ establishes a practical pathway toward higher $T_c$ hydride superconductors:

• Design principle: Deliberate insertion of elements (e.g., Sc) to promote both local electronic structure modifications (Jahn–Teller metallization of H–H bonds) and favorable extended band connectivity (MgB$_2$-like Fermi sheets).
• Theoretical blueprint: The unification of strong-coupling localized modes with delocalized bonding on the Fermi surface, creating isotropic superconductivity.
• Open questions: The role of lattice instability and dehydrogenation below $\sim$195 GPa, detailed phonon dispersion and $N(0)$ under decompression, and extension to other multinary or lower-pressure systems.

LaSc$_2$H$_{24}$ thus stands as a model compound for the experimental and theoretical exploration of ambient-condition superconductivity in polyhydrides (Song et al., 29 Sep 2025, Wang et al., 4 Jan 2026).