Synthesis And Superconducting Characterization Of Rare-earth Alloy Polyhydrides Under High Pressure

Achieving room-temperature superconductivity has been a century-old dream of mankind since the discovery of superconductors.In 2004,Ashcroft proposed that hydrogen-based metal alloys are potential superconductors,which attracted widespread attention.As a fundamental thermodynamic parameter,pressure can change the physical and chemical properties of materials,transform non-metals into metals,and form high-pressure phases with new structures and properties.In recent years,a breakthrough has been made in the field of superconducting hydrogen-rich compounds by using high-pressure technology.A series of ionic hydrogen clathrate superconductors represented by La H10 proposed by theoretical predictions have been confirmed experimentally and created a superconducting temperature record,which has great scientific significance.However,the pressure required for synthesis of these highTc superconducting hydrides currently remain extremely high.Therefore,the next challenge is to achieve high-Tc superconductivity at significantly lower pressure,with a clear final goal of reaching ambient pressure.Compared with binary hydrides,polyhydrides provide more abundant structures resulting from diverse chemical compositions,and synergistically handle the merits of different elements.Recent theoretical studies have shown that by combining two or more types of binary hydrides with similar structural features,it is possible to form polynary hydrides with similar structural properties and higher stability.In rare earth alloys,due to the similar electronegativity,electronic configuration and atomic radius of rare earth elements,a solid solution with a disordered structure will be formed,which provides a feasible way for us to explore the influence of multi-element regulation on superconducting properties experimentally.Therefore,there is an urgent need to conduct more experimental exploration on alloy hydrides to obtain high-temperature superconductors under high pressure.In this paper,three rare-earth alloy material systems,La-Y-H,La-Ce-H and LaCe-Y-H,were selected and combined with high-pressure in-situ electrical measurement technology and synchrotron radiation X-ray diffraction technology to conduct in-depth high-pressure experimental research.The innovations results are as follows:1.The hydrogen-dominant metallic compound,namely superhydride,was proposed as a promising candidate for high-temperature superconductor due to“chemical pre-compression” that could significantly lower the metallization condition to experimentally reachable pressure.However,apart from Ce H9/10,the synthesis pressures of the above-mentioned hydrides remain too high,which severely hinders the in-depth investigation and potential applications of hydrides.In addition to further increasing Tc,apparently,how to reduce the synthesis and stabilization conditions of remarkable superconducting hydrides down to a relatively lower pressure has been another challenging issue in the field.In this work,we experimentally investigated the crystal structure,superconductivity,and especially,the stability range of pressure of the La-Y alloy tetrahydride.Owing to the introduction of La and the important effect of configuration entropy,substitutional(La,Y)H4 with tetragonal I4/mmm symmetry was successfully synthesized at about 110-120 GPa & 2,200 K and can be preserved to at least 80 GPa during decompression.Its superconductivity with Tc up to ~100 K has been clarified by the phenomena of reproducible zero resistance and magnetic suppression.The stabilization pressure of this specimen is significantly reduced,as well as the extrapolated upper critical field of 35-47 T(about 2 times as high as that of YH4),implying that the superconducting properties of ternary(La,Y)H4 are somewhat improved compared with binary YH4.Present results on La Y alloy hydrides show that the alloy hydride is an ideal superconductor with excellent properties,and it also provides a new idea and experimental basis for searching for high-temperature superconductors at lower pressures.2.Since the first discovery of mercury superconductivity,the search for roomtemperature superconductors has been one of the most important topics in the field of condensed matter physics.As the most promising candidate for high-temperature superconductors,compressed metallic hydrogen-rich compounds have achieved successive breakthroughs in superconducting temperature records,especially the Tc value of 250-260 K observed in the clathrate hydride La H10.Based on these featured superhydrides,how to further tune and improve the superconducting properties,while taking into account their synthesis pressures,is one of the most important challenge issues in the field.The authors propose that if half of the weakly magnetic Ce in Ce H9 is replaced by non-magnetic La,Abrikosov-Gor’kov suppression-like effect of Ce should be significantly weakened,resulting in the giant lift of Tc in(La,Ce)H9 over that in Ce H9.In this paper,the ternary alloy hydride P63/mmc-(La,Ce)H9 was synthesized at high temperature and pressure by using La Ce alloy(molar ratio 1:1)and NH3BH3 as precursors.In experiments we discovered a giant Tc enhancement in equal-atomic substitutional ternary alloy P63/mmc-(La,Ce)H9 that exhibits high Tc values of 148-178 K in the pressure range of 97-172 GPa examined in this work.These Tc values are significantly higher,by up to 80 K,compared to the results found in binary Ce H9,which represents a giant,80% enhancement.The present results reveal that multi-element synergistic regulation is an effective way to optimize high-temperature superconducting materials,and provides guidance for future experiments to synthesize hydride superconductors with high Tc at low pressure.3.The introduction of additional elements can increase the degree of freedom of the hydride system,which provides a richer variety of structural prototypes for the screening of superconducting hydrogen-rich materials,and brings new opportunities for the continued search for high-temperature superconductors.Recent studies have shown that superhydrides can hold high Tcs in solid solutions at relatively low stabilized pressure.Since the rare-earth metals(La,Ce and Y)have similar electronegativities,electron configurations,and atomic radii,they can easily form disordered solid solution alloys.Furthermore,La,Ce,and Y hydrides are the three most well-known superconducting binary hydrides,and they also have excellent superconducting properties at high pressure.Therefore,we used La Ce Y(molar ratio1:1:1)alloy and NH3BH3 as precursors to explore the La-Ce-Y-H system electrical experiments under high-pressure.The experimental results show that the(La,Ce,Y)Hx has a superconducting transition temperature of 112-165 K at a pressure of 107-164 GPa.By further studying the dependence of superconducting transition on external magnetic field,we estimated that(La,Ce,Y)Hx possess an upper critical magnetic field of 110 T at T= 0 K.In addition,in response to some questions raised by Hirsch et al.about superconducting hydrides,we conducted additional analysis on the variation of superconducting transition broadening with magnetic field,and found that hydride superconductors(La,Ce,Y)Hx conform to the characteristics of traditional superconductors,deepening the understanding of hydride superconductors under high pressure,providing a reference for experimental research on more complex multicomponent superconducting hydride systems.