Modulation And Mechanism Of Nonhydrostatic Pressure On The Structure And Physical Properties Of Several Typical Hydrogen-rich Compounds

Superhydrides generally refer to a class of complex compounds containing six or more hydrogen atoms per metal atom in each formula unit,and superconductivity inthrese compounds has gained broad attention from the international scientific communities in the past decade,attracting great interest from scientists in many fields such as physics,chemistry,materials science.Pertinent works have promoted development of advanced ultrahigh-pressure synthesis and characterization,and showcased the ability of theory guided experimental exploration leading to major scientific discoveries.Between 2012 and2017,our own research group employed self-developed CALYPSO method and the same-named code to pioneer the prediction of a series of hydrogen-rich compounds and superhydrides,including Ca H₆,YH₆,YH₉,La H₁₀,etc.Calculations show that these compounds can realize near-room-temperature superconductivity at around 200 GPa,which is well below the pressures for producing solid metallic hydrogen.These predictions were later all confirmed by experiment.Ammonia borane has been widely used as an outstanding hydrogen source for synthesis of superhydride high-temperature superconductors.This precursor does not require a gas compression device,simplifying the experimental process and enhancing experimental efficiency.Meanwhile,the hydrogen released during the dehydrogenation of ammonia borane can directly interact with the sample,preventing the generation of surplus reactants.The accomplishments of superhydride high-temperature superconductors showcased the important role of crystal structure prediction methods in discovery of new materials with novel properties,and further stimulated in-depth studies of near-room-temperature superconductivity and the underlying mechanism.It is noted that current theoretical description of superhydride superconductors still has obvious problems and shortcomings compared with experimental observations.For example,experimentally measured superconducting transition temperature T_c can scatter over a wide range of values,and results from different batches of samples may exhibit vastly varied T_cnear the same pressure points.Such sensitive dependence of T_c on the measurement process has not been explained by theory.Moreover,there is huge systematic difference between theoretically calculated and experimentally measured T_c values,and the problem persists even after considering the anharmonic and nuclear quantum effects in the latest developed theoretical framework.What is most mystifying is the completely opposite conclusions from theory and experiment on fundamental physics issues such as crystal stability under high-pressure conditions.Such major problems and shortcomings impede the understanding of this new class of superconducting materials,even raise questions about the viability and reliability of the synthesis and characterization of certain high-profile superhydride superconductors.It is therefore imperative to carry out further theoretical analysis and operation and understanding the origin of the noted issues and come up with solutions to complement current theoretical description and build a more comprehensive and complete theory for superhydride superconductors.In the work reported in this dissertation,we considered and analyzed the synthesis and characterization process for the superhydride superconductors,and we realized that in ultrahigh-pressure experiments in the megabar regime,there is inevitably going to be omnipresent highly anisotropic nonhydrostatic stresses inside the diamond anvil and the sample chamber.Such stresses will likely cause anisotropic crystal deformation that is different from that unde hydrostatic pressure,which will further impact electronic,phonon and electron-phonon coupling properties,ultimately leading to quantitative even qualitative effects that modulate crystal stability and superconductivity.Such nonhydrostatic stress effects have not been considered in previous theoretical descriptions of superhydride superconductors.Based on this consideration,we have systematically studied in our recent work the effects of anisotropic nonhydrostatic stresses on several exemplary near-room-temperature hydrogen-rich and superhydride superconductors.Additionally,it is noted that ammonia borane,used as hydrogen source in synthesizing superhydride superconductors,is itself a hydrogen-rich compound.The unreacted ammonia borane may coexist with the synthesized superhydride sample,and it is possible that high-pressure and nonhydrostatic stresses in experiment could significantly influence the crystal structure and electronic properties of ammonia borane.Consequently,we first studied the crystal structure and the physical properties of ammonia borane under high pressure.Then,we further explored how the electronic properties of ammonia borane respond to anisotropic stresses at 300 GPa.We describe below the specific scientific problems,the approach to solve these problems,and the obtained results and conclusions of the present work.1.The study of quantitative corrections by nonhydrostatic stresses on superconducting properties of YH6 offers explanations for the high sensitivity of its superconducting transition temperature on the loading conditions and the anomalous giant systematic deviation between the theoretical and experimental T_c values.Previous experimental and theoretical works have performed broad and scientific works on the properties of YH₆ under hydrostatic pressure,and accumulated rich data and phenomena,making this compound an exemplary case for exploring the effect of nonhydrostatic stresses on superhydride superconductivity.The results of our calculations show that nonhydrostatic stresses induce anisotropic structural responses and the related changes in the electronic,phonon and electron-phonon coupling,leading to wide variations of T_c under the same hydrostatic pressure with an overall downshift.The most prominent stress induced reductions of the superconducting transition temperature occur on the deformation paths with relatively small elastic moduli,making it more likely to be encountered in experimental measurements.The present results provide reasonable explanations for the experimentally observed scattering of T_c and,in conjunction with previously reported anharmonic correction,reconcile the giant discrepancies in theoretical and experimental T_c values.These results and the associate analyses offer a more complete theoretical foundation for understanding the anomalous superconducting properties of YH₆.Meanwhile,the physical pictures and related mechanisms obtained in this work have implications for study and elucidate material properties under the complex loading conditions at ultrahigh pressures.2.The exploration of the deciding effect of nonhydrostatic stresses in achieving dynamic stabilization of the YH9 crystal structure under the experimental synthesis high pressures solves the long-standing experiment-theory dilemma.In a recent review article published in the journal Reviews of Modern Physics,theoretical condensed matter physicist and superconductivity research expert Warren Pickett from University of California at Davis ranked YH₉ among top three most outstanding hydrogen-rich and superhydride superconductors.Recent experimental studies have reported synthesis of YH₉at 170-300 GPa pressures with near-room-temperature superconductivity.But it is perplexing that existing theory pts that YH₉ is dynamically unstable in the experimental synthesis and characterization pressures,thus cannot in principle exist as a stable or even metastable phase.For this reason,a previous theoretical work ignored YH₉ in the reported study.Results from our latest study show that nonhydrostatic stresses can suppress the dynamic instability in YH₉ over the experimental synthesis and characterization pressure in a fairly broad range of strains,making it possible for the high-pressure crystal structure to stabilize.Our present work solves an important contradiction in superhydride superconductor research,which offers an explanation for the experimentally observed phenomena and,at the same time,allows for theoretical calculations in the experimental synthesis pressure range.Based on these results,we further explored superconducting properties.The obtained results show that the structural and related property responses of YH₉ to compressive stress results in a decrease of T_c with rising strain.Within a reasonable range of Coulomb pseudopotentials,the calculated T_c values of the dynamically stable YH₉crystal structure under nonhydrostatic stresses are in good agreement with the experimental data.These results solve the long-standing experiment-theory dilemma within the study of superhydride superconductors,offering a comprehensive explanation for the experimental phenomena.The physical mechanisms obtained in this study have significant implications for delving deeper into the material behavior under ultrahigh pressure complex loading environments.It also broadens the practical conditions for the stable existence of materials,offering useful insights for designing dynamically stable materials with exceptional physical properties under intricate loading circumstances.3.The analysis of the influence of high pressure and nonhydrostatic stresses on the crystal structure and physical properties of ammonia borane shows robust nonmetallicity of ammonia borane under extreme loading conditions.Ammonia borane has been widely used as hydrogen source for the synthesis of superhydrides,which necessitates an in-depth exploration of the changes in crystal structure and electronic properties of the material as it undergoes high pressure and nonhydrostatic stresses.We first used the CALYPSO structure prediction method in conjunction with first-principles calculations to theoretically predict stable structures of ammonia borane up to 300 GPa.Our findings established a new series of pressure-induced phase transition sequence.Electronic structure calculations disclosed a decrease in the electronic bandgap with rising pressure in ammonia borane,which nevertheless maintained its nonmetallic nature up to 300 GPa.Subsequently,we selected the P-1 structure with the smallest band gap and explored its response to anisotropic stresses at 300 GPa.The results indicate that relatively minor changes in the bonding and charge distribution within the structure of ammonia borane are insufficient to close the band gap under compressive or shear stresses.Ammonia borane maintains robust nonmetallicity under extreme nonhydrostatic stresses.Therefore,even the potential presence of unreacted starting material ammonia borane in the experiment will not interefere with electrical transport measurements on superhydride superconductors.These findings answered the question of whether the electronic properties of ammonia borane would change under high-pressure experimental environments,establishing its fundamental physical properties in ultrahigh-pressure scenarios.Meanwhile,the insights gained here may help understand and design molecular solids with tailored mechanical and transport properties.In summary,this dissertation offers a comprehensive account of our recent studies on the crucial impact of nonhydrostatic stresses on properties of two representative prototypical superhydride superconductors YH₆ and YH₉.The obtained results explain anomalous quantitative deviations of the theoretical results from the measured experimental data,solve a long-standing dilemma on theory-experiment contradiction,and showcase nonhydrostatic stresses as a general approach to modulating a new class of superconducting materials.Meanwhile,we explored the crystal structure and electronic properties of ammonia borane,which has been widely used as experimental hydrogen source,under high-pressure and nonhydrostatic stress conditions.This investigation affirmed that the electronic properties of ammonia borane do not interfere with experimental measurements and established it as a suitable hydrogen source for synthesizing ultra-hydrogen-rich superconductors,and established robust nonmetallicity of ammonia borane under extreme loading conditions.The conceptual considerations and computational and analytical methods devised,developed and implemented here are expected to stimulate and promote broad ultrahigh-pressure materials physics research.