Under extremely high pressure, hydrogen will be transformed into metallic hydrogen due to its unique quantum and electronic structure, and it is very likely to become a room temperature superconductor. Previously, Harvard University reported the experimental evidence of the transformation of solid hydrogen into atomic metal hydrogen under the pressure of 495GPa, but it was controversial. In addition to realizing atomic metallic hydrogen in pure hydrogen system, adding other elements into hydrogen system to form rich hydride with high hydrogen content is also a way to obtain metallic hydrogen: using "chemical precompression" of non-hydrogen atoms to realize metallic state and superconductivity at lower pressure. Therefore, it is an important research direction in the field of hydrogen-based superconductivity to find hydrogen-rich compounds with higher hydrogen content and excellent properties. A new barium super hydride BaH12 (pseudocubic CMC 2 1) was successfully synthesized by doping the hydride BA with the highest hydrogen stoichiometry in the experiment. Based on in-situ high-voltage synchrotron radiation X-ray diffraction and theoretical calculation, it is found that the new barium super hydride BaH 12 has a twisted cubic structure and is metallic. BaH 12 can be stabilized to 75GPa, which is about one-fifth of the pressure required for molecular metal hydrogen. In-situ high voltage measurement shows that the superconducting transition temperature is 20K at 140GPa. This study has made landmark progress in understanding the structural characteristics of hydrogen-based high temperature superconductors, which provides an important reference way for realizing room temperature superconductivity at atmospheric pressure.
By in-situ high-voltage synchrotron radiation X-ray diffraction measurement technology combined with laser heating technology, we successfully synthesized hydrogen-rich compound BaH 12 with quasi-cubic structure in four groups of DAC #B0-B3 experiments (see figure 1). In order to overcome the problem of hydrogen permeation, we use NH3 BH 3(AB) as the hydrogen source. This method has been proved to be effective in synthesizing super hydride under the pressure of more than one million atmospheres. Metals Ba (0 ~ 10 micron) and AB (as hydrogen source and pressure transmission medium) are heated by laser to above 1500K under target pressure. After heating, the volume expansion of the metal sample can be clearly seen, which indicates that new compounds are generated. The stoichiometric ratio and symmetry of new barium hydride were further determined by combining experimental measurement with theoretical calculation.
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In addition to the structure, through further theoretical calculation, several twisted cubic structures Bah12 are proposed: CMC 21,P 2 1, P 1 (see Figure 2). The calculation results show that,
-BaH 12 is unstable in thermodynamics and kinetics (Δ hform >: 0.19ev/atom), so it may be distorted into Cmc 2 1, P 2 1 or P 1 with low symmetry, and the diffraction peak of weak non-cubic phase on XRD diagram is also. Combined with the metal properties observed in the experiment, the synthesized BaH 12 has the structure of Cmc 2 1.
Fig. 2: Phonon and electron density diagrams of BaH 12 with several twisted cubic structures.
The calculation results of electron local distribution function show that the hydrogen atom in Cmc 2 1-BaH 12 structure is H2(dH-H=0.78? ) and
(dH-H=0.8 1 and 1.07? ), forming a horseshoe-shaped long chain (DH-H.
Fig. 3: electronic local function and density of States distribution of CMC 21-bah12 structure under 3: 150 GPa pressure.
In order to study the metallicity and superconductivity of Cmc 2 1-BaH 12 structure, we made in-situ high-voltage electrical measurements. After the target samples were prepared under the conditions of 140 GPa and 1600 K, the in-situ high-voltage resistance measurement results confirmed their metallic properties, and the superconducting transition temperature at 140 GPa was 20 K, which was consistent with the theoretical results (Figure 4). BaH 12 is the metal hydride with the highest hydrogen content and the lowest stable pressure, which provides a new idea for the superconducting research of metal hydrogen and hydrogen-rich compounds.
Fig. 4: Variation of sample resistance with pressure at 4:140 GPA and calculation of superconducting parameters of BaH 12.
* * * The first authors of this study are Dr. Chen from Jilin University, Dr. Dmitrii V. Semenok and Dr. Alexander G. Kvashnin from Skolkovo Institute of Technology, and the corresponding authors are Professor Cui Tian from Ningbo University, Professor Jilin University and Professor Artem R. Oganov from Skolkovo Institute of Technology. This work is strongly supported by the National Natural Science Foundation of China and the synchrotron radiation line BL 15U 1 of Shanghai Light Source.
Paper link:
/articles/s 4 1467-020-20 103-5