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 Tc 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 studies21,22, 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 15.9537 and 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 Tc, we proceeded to regulate the pressure dependence on Tc, as shown in Fig. 3. With the increase of pressure, Tc increases first and then tends to be flat. The highest Tc of 178 K in this work was observed at 172 GPa (Supplementary Fig. S4-S9). In addition, the pressure dependence of Tc 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 capabilities10,20. 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 Tc. 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 Tc by about 12 K. The extrapolated values of the upper critical field µ0Hc2(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.