Giant enhancement of superconducting critical temperature in substitutional alloy (La,Ce)H9

doi:10.21203/rs.3.rs-1653407/v1

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Giant enhancement of superconducting critical temperature in substitutional alloy (La,Ce)H₉

https://doi.org/10.21203/rs.3.rs-1653407/v1

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A sharp focus of current research on superconducting superhydrides is to raise their critical temperature T_c at moderate pressures. Here, we report a striking discovery of giant enhancement of T_c in CeH₉ obtained via random substitution of half Ce by La, leading to equal-atomic (La,Ce)H₉ alloy stabilized by maximum configurational entropy, containing the LaH₉ unit that is unstable in pure compound form. The synthesized (La,Ce)H₉ alloy exhibits T_c of 148-178 K in the pressure range of 97-172 GPa, representing up to 80% enhancement of T_c compared to pure CeH₉ and showcasing the highest T_c at sub-megabar pressure among the known superhydrides. This work demonstrates substitutional alloying as a highly effective enabling tool for substantially enhancing T_c via atypical compositional modulation inside suitably selected host crystal. This optimal substitutional alloying approach opens a promising avenue for synthesis of high-entropy multinary superhydrides that may exhibit further increased T_c at even lower pressures.

Synthesis of ternary (La,Ce)H₉ alloy superhydride. We took a flake of La-Ce alloy sample from a homogeneous region of the inside part of the specimen and loaded it into a diamond anvil cell (DAC) together with NH₃BH₃ as the pressure transport medium and hydrogen source. The desired mole ratio of La: Ce was determined to be ~1:1 by energy dispersive X-ray (EDX) spectroscopy (Supplementary Fig. S1). For synchrotron X-ray diffraction (XRD) and T_c measurements on the synthesized La-Ce superhydride, a total of seven samples were compressed to 110-120 GPa at room temperature and then heated to 2,100 K with pulsed radiation from an yttrium-aluminum-garnet laser (Supplementary Table S1). The color of the sample changed significantly after laser spot irradiation (Fig. 1a), indicating that the expected chemical reaction occurred. After keeping the synthesized samples under high pressure while quenching to room temperature, we performed subsequent structure and superconductivity characterizations.

Crystal structure of synthesized (La,Ce)H₉. To determine the crystal structure of synthesized La-Ce alloy superhydride, we conducted in-situ XRD experiments in synchrotron radiation sources. The representative XRD pattern of the product in cell-4 at 110 GPa is shown in Fig. 2a. The observed peaks can be indexed by an hcp lattice with cell parameters of a = 3.76 Å and c =5.68 Å (Supplementary Table S2). The weak peak marked with an asterisk is from the tetragonal tetrahydride reported in previous studies^21,22, which is a common occurrence caused by temperature or pressure gradients during high-temperature-high-temperature synthesis. A similar hcp structure mixed with tetrahydride was successfully reproduced at 115 GPa in another independent experimental run (Supplementary Fig. S2), corroborating the reliability of this structure. Although the occupancy details of the hydrogen atoms cannot be determined experimentally due to the weak X-ray scattering cross section, the measured unit cell volume can be used to estimate the hydrogen concentration of the superhydride, and the ratio of H/metal was estimated to be ~ 9 through the lattice volume expansion from 15.95³⁷and 14.01²³ Å³/f.u. (elemental La and Ce lattice) to 34.80 Å³/f.u.. Furthermore, the fitted equation of state (EOS) during decompression is highly consistent with the simulated EOSs (Supplementary Fig. S3), where the experimental points lie between the lines of hcp CeH₉ and hypothetical hcp LaH₉. Combined with the composition of the original alloy sample, our diffraction experiments show that ~50% of the Ce atoms in the recently discovered structure of P6₃/mmc-CeH₉ are replaced by La, forming a unique ternary alloy superhydride P6₃/mmc-(La,Ce)H₉ (Fig. 2b).

Superconductivity in (La,Ce)H₉. To probe superconductivity in the synthesized alloy superhydride, we performed electrical transport measurements. Representative measured electrical resistance data as a function of temperature are shown in Fig. 1b, which clearly shows the superconducting transition, as indicated by the sharp drops in resistance occurring at 158 K, 168 K, and 173 K at about 110 GPa, 125 GPa, and 110 GPa, respectively. In these experiments, zero resistance states were observed in cell-1 and cell-4 (Fig. 1b inset), excluding the possibility that the abrupt drop of resistance on cooling arises from structural transitions. To determine the highest value of T_c, we proceeded to regulate the pressure dependence on T_c, as shown in Fig. 3. With the increase of pressure, T_c increases first and then tends to be flat. The highest T_c of 178 K in this work was observed at 172 GPa (Supplementary Fig. S4-S9). In addition, the pressure dependence of T_c varies slightly in different experiments, which may originate from subtle differences in molar ratios of the La-Ce alloy samples or different degrees of anisotropic stress during compression/decompression that leads to variable deformation of the lattice in different experiments³⁸. With decreasing pressure, the superconducting transition tends to disappear at about 90 GPa (Supplementary Fig. S6), indicating a probable decomposition of the superconducting phase.

Due to the rather small size of the samples, it is impractical to detect weak signals of the magnetic flux expulsion (i.e., Meissner) effect with current experimental capabilities^10,20. However, due to the Pauli paramagnetic effect of electron spin polarization and the diamagnetic effect of orbital motion, an applied external field can disrupt the Cooper pairs, thereby reducing the value of T_c. As shown in Fig. 4a, the resistance drop gradually shifts to lower temperatures as the magnetic field increases in the range of 0–9 T at 110 GPa. The upper critical field as a function of temperature, defined as 90% of the resistance, is shown in the inset of Fig. 4b. At μ₀H = 9 T, the application of the magnetic field reduces T_c by about 12 K. The extrapolated values of the upper critical field µ₀H_c2(T) and the coherence lengths towards T = 0 K are 56 T and 24 Å and 76 T and 21 Å, respectively, by Ginzburg-Landau (GL)³⁹ and Werthamer-Helfand-Hohenberg (WHH)⁴⁰ model fits. The magnitude of the short coherence lengths and high upper critical fields indicate that ternary (La,Ce)H₉ is a typical type-II superconductor.

The clathrate REH₉ phase with H₂₉ cage was predicted⁸ to be a high-temperature superconductor in 2017, and this structural prototype has since been experimentally verified by a series of RE metal superhydrides^41-44. Of particular interest among these compounds is CeH₉ that was synthesized at easily achievable megabar pressure and exhibited an experimental T_c of ~95 K²¹. Compared to the higher-T_c LaH₁₀^9,10,36, the low T_c of CeH₉ stems from its low logarithmic average phonon frequency, which may be related to the 4f electrons in Ce at high pressure²³. Furthermore, no superconductivity or only low-temperature superconductivity was observed in other similar RE metal superhydrides, where strongly correlated interactions of 4f electrons dramatically suppress the superconductivity^41-45. This analysis points to rational improvement of superconductivity in CeH₉ via substitution of Ce by La that hosts no 4f electron but otherwise shares most chemical characters.

Although hcp LaH₉ is calculated to be thermodynamically unstable and has not been observed experimentally among binary systems at the corresponding pressure of ~100 GPa, the LaH₉ structural unit appears in synthesized ternary La-Ce alloy superhydride via La-substitution modification of the native CeH₉ units. This result can be elucidated by the Hume-Rothery rule⁴⁶, because the ratio of La/Ce atomic radii (~1.048 at 100 GPa)^23,37 is well within the rule’s stated 15% range, and La and Ce have similar electronegativity values³³, thus satisfying the conditions for the formation of a disordered solid solution compound⁴⁶. Moreover, laser heating helps to overcome the kinetic reaction barrier and stabilize substitutional (La,Ce)H₉ via the contribution of the configurational entropy to the total free energy, overcoming the contribution of mixing enthalpy and the energy increment of LaH₉ relative to LaH₁₀ at a sufficiently high temperature³⁴. In particular, the configurational entropy reaches the maximum in an equal-atomic distribution and plays a significant role in stabilizing ternary (La,Ce)H₉. This effect should be more pronounced in higher multinary superhydrides, such as equiatomic medium- or high-entropy alloy superhydrides. Further theoretical examination and experimental verification are needed.

In conclusion, we discovered a giant T_c enhancement in equal-atomic substitutional ternary alloy (La,Ce)H₉ that exhibits high T_c values of 148-178 K in the pressure range of 97-172 GPa examined in this work. These T_c values a significantly higher, by up to 80 K, compared to the results found in binary CeH₉, which represents a giant, 80% enhancement. This ternary alloy superhydride integrates the native CeH₉ structural units and the atypical LaH₉ units that are unstable in pure compound form but stabilized by optimal configurational entropy under the laser-heated DAC compression synthesis conditions. These results show that substitutional alloying is an effective and promising approach to tune and improve superconducting superhydrides, leading to the long-sought scenario of high-temperature superconductivity at relatively lower pressures. The present findings also provide insights for expanding the scope of ongoing studies in search of room-temperature superconductors among diverse compounds via construction of multinary alloys via suitable metal-atom substitution.

Alloy preparation. La-Ce alloy (1:1 in molar ratio) was prepared by levitation melting technology, where mixed elemental La and Ce with purity of 99.7% were heated to completely melt in an Ar atmosphere at a pressure of 60 KPa for 1-2 minutes and then were annealed for about 40 minutes. To ensure the uniformity of the composition, the specimen ingot needed to be turned over and the above process was repeated more than twice.

Synthesis in diamond anvil cell. We synthesized the superhydrides via a reaction of La-Ce alloy and NH₃BH₃ (Sigma-Aldrich, 97%) in diamond anvil cells (DACs). The diamond anvils with culets of 60-100 µm in diameter were beveled at 8.5^o to about 250 µm in diameter. A composite gasket consisting of a rhenium outer annulus and an aluminum oxide (Al₂O₃) epoxy mixture insert was employed to contain the sample while isolating the electrical leads in the electrical measurements. La-Ce alloy foil with a thickness of 2-3 µm was sandwiched between NH₃BH₃. Sample preparation and loading were done in an inert Ar atmosphere with residual O₂ and H₂O contents of < 0.01 ppm to guarantee that the sample was properly isolated from the surrounding air atmosphere. The samples were compressed to target pressures at room temperature. The pressure values in cells were determined from room-temperature first-order diamond Raman edge calibrated by Akahama^47,48. One-side laser-heating experiments were performed using a pulsed YAG laser (1064 nm) with spots of approximately 10 μm in diameter. The temperature was determined using the emission spectrum of the black body radiation within Planck’s radiation law.

Structure characterization. X-ray diffraction (XRD) patterns were obtained at BL15U1 (λ = 0.6199 Å) of Shanghai Synchrotron Radiation Facility and BL10XU (λ = 0.4131 Å) of at the SPring-8 facility⁴⁹ with focused monochromatic X-ray beams (5 × 12 and 3 × 2 µm²). A Mar165 CCD detector and an imaging plate detector (RAXIS-IV; Rigaku) were used to collect the angle-dispersive XRD data. The sample to detector distance and other geometric parameters were calibrated using a CeO₂ standard. The software package Dioptas was used to integrate powder diffraction rings and convert the 2-dimensional data to 1-dimensional profiles⁵⁰. The full profile analysis of the diffraction patterns and the Rietveld refinements were done using GSAS and EXPGUI packages⁵¹.

Electrical transport measurement. The resistance was all measured via the four-probe van der Pauw method where four Au electrodes were placed on the NH₃BH₃ plate with currents of 1-100 µA. The temperature dependence of the electrical resistance was measured upon cooling and warming cycles with a slow temperature ratio (0.2 K·min⁻¹). The data was taken upon warming, as it yielded a more accurate temperature reading. Symmetric Cu-Be alloy DACs were used for electronic transport measurements under external magnetic fields up to 9 T.

Data availability

The authors declare that the main data supporting the findings of this study are contained within the paper and its associated Supplementary Information. All other relevant data are available from the corresponding author upon reasonable request.

Acknowledgements

We thank Professor Changfeng Chen for a critical reading and extensive refining of the paper. This research was supported by National Key Research and Development Program of China (Grant No. 2021YFA1400203), the National Natural Science Foundation of China under Grants No. 12074139, 12074138, and 52090024; Interdisciplinary Integration and Innovation Project of Jilin University (JLUXKJC2020311), Fundamental Research Funds for the Central Universities (Jilin University, JLU), Program for JLU Science and Technology Innovative Research Team, the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB33000000). The work was also supported by JSPS KAKENHI Grant Numbers 20H05644. The XRD measurements were performed at the BL15U1 at Shanghai Synchrotron Radiation Facility and the BL10XU of SPring-8 (Proposal No. 2021A1172 and 2021B1407).

Author contributions

G. Liu designed the research. Y. Ma supervised the research. J. Bi, Nakamoto, and K. Shimizu collected and analyzed the synchrotron X-ray diffraction data. J. Bi performed the electrical transport measurements and processed the experimental data. J. Bi, G. Liu, and Y. Ma wrote the paper. All authors contributed to discussing the results and writing the paper.

^*[email protected]

Competing interests

The authors declare no competing interests.

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Giant enhancement of superconducting critical temperature in substitutional alloy (La,Ce)H₉

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