| The extreme conditions of high pressure can decrease the distance between atoms signally,leading to stronger interactions among atoms.Pressure would have a profound effect on the nature of the material,accompanying with formation of high pressure phase with new structure and new property,that is absent in the normal conditions.Hydrogen,as the simplest element,is predicted to become metallic phase under ultrahigh pressure.According to the prediction,this metallic phase is also one kind of room temperature superconductor.So there have been wide investigations on metallic hydrogen,which is discribed as “the holy grail” of high pressure physics.The exploration of metal hydrogen has continued for more than eighty years so far.Recently,one research group of Harvard university published their achievement on Science,declaring that they have detected the sigh of metallic hydrogen under the pressure of 495 GPa.However,this controversial result still needs further verification,and this high metallic pressure has increased the difficulty of high-pressure further exploration seriously.So people carry out some researches in another way in order to obtain some regular understanding of metallic hydrogen at lower pressure condition.These researches mainly include two aspects.The first approach is based on the chemical pre-compression effect proposed by Ashcroft at 2004.The heavier atom introduced in the lattice of hydrogen will produce compression effect on the hydrogen atoms,which is predicted to decrease the metallization pressure of hydrogen.The second is researches on halogen elements whose physical and chemical properties are similar to hydrogen elements.Studies on the metallization of halogen elements and their compounds and the mechanism of high pressure transition will be helpful to understand the dissociation and metallization of hydrogen under high pressure.The topic of this paper is based on the two aspects of researches mentioned above.We have carried out research on the system of calcium-hydrogen and magnesium-hydrogen,which including behaviors of Mg+H2 and Ca+H2 under high pressure high temperature conditions and high pressure structural transition of the ordinary stoichiometric Mg H2.This systematic research will be helpful to understand the chemical pre-compression effect,and guide the exploration of metallic hydrogen.In addition,xenon difluoride is a typical noble gas fluoride.In order to obtain the physical image helpful to study on metallic hydrogen,we have adopted high pressure experiment technique and first principle calculations to investigate the high pressure structure transition and metallic phenomenon of Xe F2.In summary,we have get the following innovative results.1.Calcium polyhydride is synthesized from elements at 50 GPa and temperature above 1900 K experimentally for the first time.By analyzing results of Raman and XRD measurement up to 70 GPa,This polyhydride is inferred to be the untraditional stoichiometry Ca H4 which is predicted to be the ground state of Ca-H system at 50 GPa in former theoretical study.Among all of the metal hydrides with unconventional stoichiometry observed under high pressure,Ca H4 is the only one with the metal: hydrogen ratio of 1:4.The zero-pressure bulk modulus B0 of Ca H4,obtained from fitting to a Birch-Murnaghan equation of state is 25.7(4)GPa.The decomposition of Ca H4 is observed at 28 GPa in Raman spectra.The crystal structure of Ca H4 with I4/mmm symmetry includes two ionic hydrogen and one elongate H2 unit,which has the similar length with hydrogen molecular at higher pressure.According to the variation trend of H2 unit in Ca H4,this unit is suggested to dissociate into metallic phase at more moderate pressure than pure hydrogen.In this respect,Ca H4 could be served as the chemical pre-compression material,which is regarded as the carrier of metallic hydrogen.This study will attribute to realizing chemical pre-compression effect and exploring metallic hydrogen.2.Researches on Mg-H system are in two two aspect,including the study on high pressure structure phase transition of Mg H2 and the synthesis of magnesium hydride with unconventional stoichiometry.We have confirmed the phase sequence of magnesium hydride under pressure.Mg H2 is stable as the coexist of ? phase and ? phase before 11.2 GPa,after which it transforms into a Pbc21 phase.The Pnma phase takes the place of Pbc21 phase at 17 GPa.These two phase transitions are corresponding with 13% and 10% volume collapse respectively.Our results solve the controversy on high pressure structures of Mg H2,and provide comparative data for the following exploration on magnesium and hydrogen.In the study on magnesium and hydrogen under high pressure and high temperature,the sample is heated to 2000 K by laser at 42 GPa and 55 GPa.There is change on the morphology of magnesium after laser heating.Results of Raman and XRD both indicate the formation of magnesium hydride(Mg H2).The synthesis of magnesium hydride with unconventional stoichiometry still need higher pressure and temperature.3.It has been reported that xenon difluoride will undergo metallization transition under high pressure,accompanying with abundant structural phase transition.But both crystal structure and phase transition sequence of Xe F2 remain controversial under high pressure.We have investigated the structural phase transition and band gap change of Xe F2 by means of Raman spectra,absorption spectra and synchrotron XRD,combined with DFT calculations.The I4/mmm structure which is predicted to be stable before 82 GPa,collapse into Immm at 28 GPa.Simultaneously,the double degenerate Raman peak slip into two individual peaks,and a symmetry forbidden electron transition is observed in absorption spectra.At 59 GPa the predicted high pressure phase(Pnma structure)is definitely confirmed in experiment for the first time.In our study,non-hydrostatic conditions significantly reduce the phase transition pressure and the lattice symmetry.We propose that non hydrostatic effect in sample chamber is response for the discrepancy between the experimental research and theoretical calculations on Xe F2.A reduction of the band gap is indicated by the optical absorption experiment,accompanying with the sample color getting dark as pressure increase.The metallization pressure is estimated to be 152 GPa.4.In the experimental method,we have built a hydrogen gas loading system independently.The method of sample loading in laser heating experiment has been optimized,which has been used to synthesis Re N2. |
PDF Full Text Request