The Phase Stability And Structure Characteristics Of Several High-pressure Materials

High-pressure materials have attracted extensive attention due to their unique physical and chemical properties.By adjusting the pressure and temperature,the atomic and electronic structures of high-pressure materials are reconstructed,leading to the metal-insulator transition(MIT)or even the superconducting transition of materials,which is of great significance to the development of optoelectronic materials and room-temperature superconducting materials.However,it still presents a fundamental challenge to character the structures of high-pressure materials,which seriously hinders the understanding of their MIT and superconductivity mechanisms and the further design of new materials.On the one hand,the conventional structural characterization methods have many shortcomings in describing the structures of high-pressure materials:(1)X-ray diffraction(XRD)cannot effectively detect the positions of hydrogen atoms,(2)neutron diffraction can only characterize the structures at low pressure,and(3)infrared and Raman spectroscopies are insufficient to determine the atomic structures of materials,etc.On the other hand,the incomplete theoretical understanding of the factors affecting the phase stability has also triggered widespread controversy regarding the phase diagrams of high-pressure materials.To adress the above issues,we systematically study the phase stabilities and structure characteristics of several high-pressure materials to explore new structure-differentiation means and investigate the influencing factors of phase stability.Firstly,we study the phase stability and structure characteristics of high-pressure hydrogen.The calculations of nonlocal electron exchange,van der Waals(vdW)interactions and zero-point energy indicate that they have great influence on the transition pressures between different phases,but little effect on the structure differentiation of the same phase.Correspondingly,we study the dielectric constants of the high-pressure hydrogen,and find that the anisotropic dielectric constants of the different hydrogen solids and their responses to pressure behave differently depending on the atomic structures,especially for the candidates of phases II and III.These findings are robust regardless of the quantum and thermal motion of hydrogen solids.Therefore,the anisotropic dielectric property can serve as a potential measure for probing the structures of high-pressure hydrogen.Similar to high-pressure hydrogen,the conventional means cannot effectively characterize the precise atomic structures of high-pressure hydrides.Herein,we propose to use Nuclear Magnetic Resonance(NMR)spectroscopy to effectively detect its structures.We study the NMR parameters of H3S and La H10 and find that the different candidate structures of H3S(or La H10)exhibit significant differences in the electric field gradient(EFG)tensors of the ³³S(or ¹³⁹La)sites,which indicates that NMR spectroscopy can well capture the structure differences between these structures,even the small changes in atomic positions.Therefore,NMR spectroscopy can be used to effectively probe the structures and phase transitions of H3S and La H10.Our results clarify the relationship between the EFG tensor parameters and the atomic structures and provide a potential means to detect the structures of high-pressure hydrides.Based on the fact that the magnitudes of vdW interactions are positively related to the metallic properties of the structures in high-pressure hydrogen and high-pressure hydrides,we study the role of vdW corrections in the MIT of transition metal oxides(TMOs).We calculate the pairwise and screened vdW energies,respectively,and find that different TMOs systems exhibit distinct vdW interactions,which are mainly related to their structures.Contrary to other TMOs systems,the long-range electrostatic screening interactions stabilize the metal phase of CaFeO3,which stems from its singular C6 coefficients in Fe atomic sites.Our calculations indicate the significant role of vdW forces in the MIT of TMOs and lay the foundation for the phase-stability study of other TMO systems.