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Following the discovery of high-temperature superconductivity in the La–H system, we studied the formation of new chemical compounds in the barium-hydrogen system at pressures from 75 to 173 GPa. Using in situ generation of hydrogen from NH 3 BH 3 , we synthesized previously unknown superhydride BaH 12 with a pseudocubic ( fcc ) Ba sublattice in four independent experiments. Density functional theory calculations indicate close agreement between the theoretical and experimental equations of state. In addition, we identified previously known P 6 /mmm -BaH 2 and possibly BaH 10 and BaH 6 as impurities in the samples. Ab initio calculations show that newly discovered semimetallic BaH 12 contains H 2 and H 3 – molecular units and detached H 12 chains which are formed as a result of a Peierls-type distortion of the cubic cage structure. Barium dodecahydride is a unique molecular hydride with metallic conductivity that demonstrates the superconducting transition around 20 K at 140 GPa. Metallization of pure hydrogen via overlapping of electronic bands requires high pressure above 3 Mbar. Here the authors study the Ba-H system and discover a unique superhydride BaH 12 that contains molecular hydrogen, which demonstrates metallic properties and superconductivity below 1.5 Mbar.

Ternary hydrides are regarded as an important platform for exploring high-temperature superconductivity at relatively low pressures. Here, we successfully synthesized the hcp -(La,Ce)H 9-10 at 113 GPa with the initial La/Ce ratio close to 3:1. The high-temperature superconductivity was strikingly observed at 176 K and 100 GPa with the extrapolated upper critical field H c2 (0) reaching 235 T. We also studied the binary La-H system for comparison, which exhibited a T c of 103 K at 78 GPa. The T c and H c2 (0) of the La-Ce-H are respectively enhanced by over 80 K and 100 T with respect to the binary La-H and Ce-H components. The experimental results and theoretical calculations indicate that the formation of the solid solution contributes not only to enhanced stability but also to superior superconducting properties. These results show how better superconductors can be engineered in the new hydrides by large addition of alloy-forming elements. Recently, high-temperature superconductivity has been reported in LaH 10 and CeH 10 . Here, the authors report superconductivity in the alloy (La,Ce)H 9-10 with T c  = 176 K at 100 GPa, providing an improved compromise between high transition temperature and low pressure requirements.

Ternary hydrides have attracted considerable worldwide attention owing to their potential high-temperature superconducting phases. In contrast to previously reported alloy-based ternary hydrides, we selected the rare earth metal yttrium and light element sulfur to form new yttrium-sulfur hydride superconductors as the target at high pressure, which also linked the two categories of binary clathrate YH 6 and covalent H 3 S high-temperature superconductors. The rare earth metal ions served as carrier donors for the dissociation of H 2 molecules and the formation of clathrate-like cage structures, while the light elements helped stabilize the materials. By applying high-pressure and high-temperature conditions, two possible ternary hydrides, i.e., and I 4/ mmm -(Y,S)H 4+δ , were discovered. is a ternary hydride with a record superconducting transition temperature ( T c ) of 235 K belonging to the single-metal hydride system, exhibiting an 11% increment in T c compared with binary . In the pressure range of 199–249 GPa, the T c of this phase displayed a decreasing tendency but with an apparent slope change, indicating the possible structural distortions or electronic structure changes at 227 GPa. Concurrently, the extrapolated upper critical magnetic field H c2 was determined to be 85 T using the Werthamer-Helfand-Hohenberg formula at 199 GPa, with a 37% increment compared with . The slight volume expansion (∼3%) of compared with that of binary signified the possible interstitial sites of sulfur atoms filling into the hydrogen cages. The size expansion of the center atoms might attract more hydrogen atoms to stabilize the hydrogen cages, contributing to the enhancement of T c and H c2 in . The present study demonstrates that introduction of light elements is an effective way for enhancing T c by forming larger hydrogen cages.

Following the discovery of high-temperature superconductivity in the La-H system, we studied the formation of new chemical compounds in the barium-hydrogen system at pressures from 75 to 173 GPa. Using in situ generation of hydrogen from NH BH , we synthesized previously unknown superhydride BaH with a pseudocubic (fcc) Ba sublattice in four independent experiments. Density functional theory calculations indicate close agreement between the theoretical and experimental equations of state. In addition, we identified previously known P6/mmm-BaH and possibly BaH and BaH as impurities in the samples. Ab initio calculations show that newly discovered semimetallic BaH contains H and H molecular units and detached H chains which are formed as a result of a Peierls-type distortion of the cubic cage structure. Barium dodecahydride is a unique molecular hydride with metallic conductivity that demonstrates the superconducting transition around 20 K at 140 GPa.

Covalent and ionic polyhydrides have become the two main camps in searching for the high-temperature superconductors under pressure. They have been considered as important platforms for exploring ternary or multiple hydrides in order to further increase the Tc or decrease the stabilization pressure. In this work, we successfully synthesized ternary hexagonal La-Ce polyhydrides stable in the pressure range of 95-130 GPa by laser-heating the La-Ce alloy (initial ratio La:Ce=2.5-3.5) in ammonia borane. Superconductivity at 176 K was strikingly preserved to about 100 GPa. The extrapolated upper critical field Hc2(0) reached 216 T at 100 GPa, the highest value among the synthesized polyhydrides. We also performed the contrast experiments and stabilized binary high-temperature superconducting LaHx with Tc-103 K at 78 GPa. In the pressure range of 95-130 GPa, the ternary hexagonal La-Ce-H system exhibits higher Tc than the binary La-H system, with the maximum difference of 100 K, and the compounds of both systems were synthesized at the same pressure and temperature conditions. These results clearly indicate that the discovered La-Ce-H system not only enriches the high-temperature superconducting hydrides but also realizes high-Tc at moderate pressures.

Among known materials, hydride superconductors have the highest critical temperatures and are very promising as a basis for electronic sensors. Superconducting quantum interference devices (SQUID), due to its unique sensitivity to magnetic fields, are the most important applications of superconductors in microelectronics. In this work, we describe a direct current SQUID made of lanthanum-cerium superhydride (La, Ce)H\\(_{10+x}\\) at a pressure of 148 GPa, with an operating temperature of 179 K and a bias current of about 2 mA. When placing (La, Ce)H\\(_{10+x}\\) in a modulated magnetic field (0.1-0.005 Hz, 5 G), we observed the generation of higher harmonics up to 18${\\nu}$$_0\\( and a periodic dependence of the sample resistance on the magnetic flux density R \\){\\propto}\\( sin(\\){\\pi}$${\\Phi}\\(/\\){\\Phi}$$_0\\(). We demonstrate that the (La, Ce)H\\)_{10+x}\\( SQUID with a size of about 6 \\){\\mu}$m, operates in the mode of low thermal fluctuations and can be used to detect magnetic fields below 0.1 G. Our findings pave the road to more advanced applications of the Josephson effect and SQUIDs made of hydride superconductors.

The discoveries of high-temperature superconductivity in H3S and LaH10 have excited the search for superconductivity in compressed hydrides. In contrast to rapidly expanding theoretical studies, high-pressure experiments on hydride superconductors are expensive and technically challenging. Here we experimentally discover superconductivity in two new phases,Fm-3m-CeH10 (SC-I phase) and P63/mmc-CeH9 (SC-II phase) at pressures that are much lower (<100 GPa) than those needed to stabilize other polyhydride superconductors. Superconductivity was evidenced by a sharp drop of the electrical resistance to zero, and by the decrease of the critical temperature in deuterated samples and in an external magnetic field. SC-I has Tc=115 K at 95 GPa, showing expected decrease on further compression due to decrease of the electron-phonon coupling (EPC) coefficient {\\lambda} (from 2.0 at 100 GPa to 0.8 at 200 GPa). SC-II has Tc = 57 K at 88 GPa, rapidly increasing to a maximum Tc ~100 K at 130 GPa, and then decreasing on further compression. This maximum of Tc is due to a maximum of {\\lambda} at the phase transition from P63/mmc-CeH9 into a symmetry-broken modification C2/c-CeH9. The pressure-temperature conditions of synthesis affect the actual hydrogen content, and the actual value of Tc. Anomalously low pressures of stability of cerium superhydrides make them appealing for studies of superhydrides and for designing new superhydrides with even lower pressures of stability.

By directly altering microscopic interactions, pressure provides a powerful tuning knob for the exploration of condensed phases and geophysical phenomena. The megabar regime represents an exciting frontier, where recent discoveries include novel high-temperature superconductors, as well as structural and valence phase transitions. However, at such high pressures, many conventional measurement techniques fail. Here, we demonstrate the ability to perform local magnetometry inside of a diamond anvil cell with sub-micron spatial resolution at megabar pressures. Our approach utilizes a shallow layer of Nitrogen-Vacancy (NV) color centers implanted directly within the anvil; crucially, we choose a crystal cut compatible with the intrinsic symmetries of the NV center to enable functionality at megabar pressures. We apply our technique to characterize a recently discovered hydride superconductor, CeH\\(_9\\). By performing simultaneous magnetometry and electrical transport measurements, we observe the dual signatures of superconductivity: local diamagnetism characteristic of the Meissner effect and a sharp drop of the resistance to near zero. By locally mapping the Meissner effect and flux trapping, we directly image the geometry of superconducting regions, revealing significant inhomogeneities at the micron scale. Our work brings quantum sensing to the megabar frontier and enables the closed loop optimization of superhydride materials synthesis.

Lanthanum (La) is the first member of the rare-earth series of elements that has recently raised considerable interest because of its unique high-Tc superhydride LaH10. Although several studies have found superconductivity and phase transitions in metallic La, there was a lack of experimental evidence for the equation of state (EoS) and superconductivity above one megabar pressure. Here, we extend the pressure range up to 140 GPa to study EoS and superconductivity of lanthanum via electrical transport and X-ray diffraction measurements. The experimental XRD patterns point to a phase transition sequences R3m-Fm3m-Fmmm above 78 GPa. All the experimental pressure-volume data were fitted by the 3rd order Birch-Murnaghan equation: V0 = 35.2 (4) A^3, B0 = 27 (1) GPa and B0' = 4. Superconducting critical temperature Tc(onset) of lanthanum is 9.6 K at 78 GPa, which decreases to 2.2 K at 140 GPa. The upper critical magnetic field Bc2(0) was found to be 0.32-0.43 T at 140 GPa. Ab initio calculations give predicted Tc(A-D)=2.2 K (mu*=0.195), dTc/dP = 0.11-0.13 K/GPa and Hc=0.4 T at 140 GPa.

Recently, several research groups announced reaching the point of metallization of hydrogen above 400 GPa. Following the mainstream of extensive investigations of compressed polyhydrides, in this work we demonstrate that small (4 atom %) doping of molecular hydrogen by strontium leads to a dramatic reduction in the metallization pressure to about 200 GPa. Studying the high-pressure chemistry of the Sr-H system at 56-180 GPa, we observed the formation of several previously unknown compounds: C2/m-Sr\\(_3\\)H\\(_{13}\\), pseudocubic SrH\\(_6\\), SrH\\(_9\\) with cubic F-43m Sr sublattice, and pseudotetragonal P1-SrH\\(_{22}\\), the metal hydride with the highest hydrogen content discovered so far. Unlike Ca and Y, strontium forms molecular semiconducting polyhydrides, whereas calcium and yttrium polyhydrides are high-Tc superconductors with an atomic H sublattice. The latter phase, SrH\\(_{22}\\) or Sr\\(_{0.04}\\)H\\(_{0.96}\\), may be considered as a convenient model of the consistent bandgap closure and metallization of hydrogen. Using the impedance measurements in diamond anvil cells at 300-440 K, we estimated the direct bandgap of the Pm-3n-like compound P1-SrH\\(_6\\) to be 0.44-0.51 eV at 150 GPa, and its metallization pressure to be 220 GPa. Together with the machine learning interatomic potentials, the impedance spectroscopy allowed us to estimate the diffusion coefficients of hydrogen D\\(_H\\) = 1.0-2.8 E-10 m\\(^2\\)/s in SrH\\(_6\\) and 1.2-2.1 E-9 m\\(^2\\)/s in P1-SrH\\(_{22}\\) at 500-600 K.