Pressure-tuned Mixed Electrons And Hydride Ions?H^-? Conduction In SrH₂

At present,hydride materials have attracted much attention over the past years for their potential to store hydrogen in the solid state.The presence of solid-state hydrogen can not only maintain high energy density,but also can be conveniently stored.The solid-state hydrogen is viewed as a kind of friendly and high-density energy materials.Compared to gaseous hydrogen and liquid hydrogen,hydrogen storage materials that storing hydrogen in solid form are more promising.The alkaline earth metal hydride?A_vH₂,A_v=Mg,Ca,Sr and Ba?is a kind of hydrogen storage material.High H^-ionic conductivity in barium hydride has been previously reported,which helps us to understand the migration and transportation of hydrogen ions in different hydrogen storage materials.In this paper,the conduction mechanism and dielectric behavior of the mixed carriers in SrH₂ is studied by high pressure in situ alternating-current impedance spectroscopy methods.Combining the results of XRD experiments and first principle calculation,the relationship between the electrical transportation properties and structure,grain,grain boundary and band gap in SrH₂has been studied.We come up with the following results:?1?In situ high pressure X-ray diffraction experiments were conducted to detect the phase transformation behaviors of SrH₂ under compression.The results show that all the diffraction peaks of SrH₂ shift from low angle to high angle with the pressure increasing.When the pressure reaches 8.6 GPa,the structure of SrH₂ is transformed from the atmospheric pressure Cotunnite?Pnma space group?phase to the high pressure Ni₂In?P6₃/mmc space group?phase.The experimental result agrees with the theoretical calculations of Jesse S.Smith et al.?2?The electronic state density,electron local function and Bader charge analysis about SrH₂ are calculated through the first principles calculation method basing on density functional theory.The calculation results show that the band gap changes discontinuously at 8.4 GPa.The?Electron localization function,ELF?diagram is used to characterize electron distribution around the ion?H^-?in the Cotunnite?Pnma space group?phase and the Ni₂In?P6₃/mmc space group?phase.We find that pressure does not change electron distribution around the ion?H^-?significantly.There is ion bond rather than covalent bond between Sr and H.?3?The electrical transport properties of SrH₂ are studied by high pressure in situ AC impedance spectroscopy.We find the mixed electrons and hydride ions?H^-?conduction in SrH₂.The maximum applied pressure is 25.3 GPa in this thesis.The total resistance decreases with the pressure increases from 0 to 8.4 GPa;the total resistance increases with the pressure increase from 8.4 to 25.3 GPa;The pressure dependent total resistance,grain resistance and grain boundary resistance of the SrH₂exhibit discontinuously changes at 8.4 GPa,which is due to the structural phase transition.The grain boundary resistance dominates in the total resistance from 0 to8.4 GPa and the bulk resistance dominates from 8.4 to 25.3 GPa.?4?By further fitting and analyzing the impedance spectrum data,we have obtained pressure-dependent electron resistance and ionic resistance in bulk and grain boundary respectively.In the bulk,ions?H^-?transport in the framework?ion channel?formed by Sr atoms.The pressure-induced ionic?H^-?transport channel narrowing makes the ion?H^-?transport more difficult in the grains,so as to the ionic resistance increase.In the bulk,the electronic transport behavior is mainly affected by the width of the band gap and the depth of the impurity/defect level.Combining with theoretical analysis,we found the main reason for the increase of the electronic resistance is attributed to the pressure-induced impurity defect level deeper below 3.4 GPa;the main reason for the decrease of electronic resistance is attributed to the pressure-induced band gap narrowing from 3.9 to 8.4 GPa.Above 8.4 GPa,the pressure-induced band gap widening leads to the electrons transport more difficult in grains accompany with the electronic resistance increase.In grain boundary,the ionic transport behavior is mainly affected by the length of ion?H^-?diffusion path.With the applied pressure increase,the ionic?H^-?diffusion path in grain boundary becomes shorter which makes transportation of ions?H^-?easier,so as to the decrease of the ionic?H^-?resistance.The electronic transport behavior in grain boundary is mainly affected by surface dangling bonds and interface defects.The enhancement of surface dangling bonds facilitates electron transfer,while the increased defects in grain boundary is not conducive to electron transfer.At the pressure below 8.4 GPa,the electronic resistance decreases because of both the increase of the dangling bond effect and the decrease of the scattering of electrons in grain boundary.At the pressure above 8.4 GPa,the dangling bond interaction at the grain boundary weakens and a large number of defects occur at the grain boundary with the increasing of pressure which makes electrons trapped and localized,so the electron transfer becomes more difficult accompany with the electronic resistance increase.Both the electronic and ionic resistances in the P6₃/mmc phase are significantly smaller than those in the Pnma phase,indicating that both the electron and ion transportation are easier in the P6₃/mmc phase.?5?There are local electron conduction in SrH₂ by analyzing the relationship between M??and Z??and f under compression.By analyzing the dielectric properties of the SrH₂,the main reason for the linear decrease of???with increasing frequency is attributed to the electron relaxation polarization;the main reason for the increase of???with increasing frequency is attributed to the dipolar relaxation polarization of ions.The polarization of the space charge enhances at grain boundary with structural change of SrH₂,the result is in agreement with the tan?-f curve.The dielectric loss of the material gradually decreases with the increase of the pressure in the P6₃/mmc phase which indicates its dielectric properties are improved.