Structures And Properties Of Typical Subgroup Metal Hydrides And Calcium Borohydride Under High Pressure

Hydrogen has the simplest electronic structure.Theoretical work predicts that solid hydrogen would transform into metallic phase at sufficient pressure,which has attractive properties such as high energy and high-temperature superconductivity.However,tremendous efforts have been devoted,but unfortuntly,solid hydrogen would not metallize at 300 GPa.Recently,metallization of solid hydrogen at 495 GPa has been reported,but this result needs to be further verified.It is still a great challenge to get such high-pressure,and it is important to reduce the pressure required to metallization.In 2004,N.W.Ashcroff proposed that non-hydrogen atoms maybe provide a chemical precompression effect inside hydrogen-rich compounds,which may potentially reduce the pressure required to metallization.Therefore,this work is aimed at the synthesis and properties of subgroup metal hydrides under high pressure.In addition,hydrogen energy as a clean energy has its own limitation of storage,we have investigated the properties of hydrogen-rich material Ca(BH4)2 under high pressure.In this work,we have investigated the synthesis and properties of Ce-H,Y-H and Ru-H systems under high pressure by synchrotron XRD and DFT calculations.A series of cerium hydrides were successfully synthesized in Ce-H,YH3 in Y-H and Ru H in RuH system.The research of these hydrides provides important experimental foundation for searching for metallic hydrogen and high-temperature superconductors in hydrides.We have investigated the Ca(BH4)2 under high pressure through synchrotron XRD and Raman spectroscopy.The obtained results are as follows:1.We have successfully synthesized a series of cerium polyhydrides directly by reaction of Ce and H2.Ce H3-Fm-3m,Ce H3-Pm-3n,Ce H4-I4/mmm,Ce H9-x and Ce H9-P63/mmc were synthesized at 3 GPa,33 GPa,62 GPa,80 GPa and 103 GPa respectively under cold-compression and these Ce polyhydrides present an increase of hydrogen content as pressure increases.β-UH3-Ce H3-Pm-3n was synthesized at 33 GPa and 1500 K in laser-heating route.The formed Ce H9 has a unique clathrate-like structure consisting of H29 cages surrounding Ce atoms occupying the hexagonal P63/mmc symmetry.Ce H9 is also with the nearest-neighbor H-H distances closest to predictions for solid atomic metallic hydrogen in all synthesized hydrides.The electron localization function of Ce H9 indicates an ionic bonding between Ce-H atoms and a weak electron localization between H-H atoms,and the band structure confirms its metallic character.The DOS calculations indicate significant contribution of H to the Fermi level.Our cold-compression experiment provides a facile route to Ce superhydrides,and the synthesized Ce H9 with atomic-like hydrogen sublattice suggests a new low-pressure rout for bulk dense atomic hydrogen stabilized by other element atoms.2.YH3 was successfully synthesized at 35.2 GPa with laser heating and it is stable to at least 70.3 GPa.We did preliminary exploration on the condition of synthesis of yttrium hydrides under high pressure in order to search for high-temperature superconductor.3.The theoretically redicted Ru H was synthesized successfully at 23.5 GPa.Ru H6 was not observed at 30 GPa and 2000 K and synthesis may require more extreme conditions.4.Joint XRD and Raman spectroscopy has been used to characterize the highpressure behavior of Ca(BH4)2,and a puzzle about high pressure structure of Ca(BH4)2 has been solved.Both XRD patterns and Raman spectra confirmed a pressure-induced phase transition of α-Ca(BH4)2.The new phase matches the predicted isoenergetic structure with C2/c space group.There is a wide phase transition region from 2.36 to 7.97 GPa in XRD experiment,and the volume collapse is 3.84%.The same phase transition was also observed in Raman spectra at 3.59 GPa.Another initial ambient phase γ-Ca(BH4)2 is stable up to 13.8 GPa.Above 13.8 GPa,the final pressureinduced amorphization occurred.