RESUMO
CONTEXT: The addition of central metal atoms to hydrogen clathrate structures is thought to provide a certain amount of "internal chemical pressure" to offset some of the external physical pressure required for compound stability. The size and valence of the central atoms significantly affect the minimum pressure required for the stabilization of hydrogen-rich compounds and their superconducting transition temperature. In recent years, many studies have calculated the minimum stable pressure and superconducting transition temperature of compounds with H24, H29, and H32 hydrogen clathrates, with centrally occupied metal atoms. In order to investigate the stability and physical properties of compounds with H cages in which the central atoms change in the same third group B, herein, based on first-principles calculations, we systematically investigated the lattice parameters, crystal volume, band structures, density of states, Mulliken analysis, charge density, charge density difference, and electronic localization function in I m 3 ¯ m -MH6 and P63/mmc-MH9 systems with different centered rare earth atoms M (M = Sc, Y, La) under a series of pressures. We find that for MH9, the pressure mainly changes the crystal lattice parameters along the c-axis, and the contributions of the different H atoms in MH9 to the Fermi level are H3 > H1 > H2. The density of states at the Fermi level of MH6 is mainly provided by H 1 s. Moreover, the size of the central atom M is particularly important for the stability of the crystal. By observing a series of properties of the structures with H24 and H29 cages wrapping the same family of central atoms under a series of pressures, our theoretical study is helpful for further understanding the formation mechanism of high-temperature superconductors and provides a reference for future research and design of high-temperature superconductors. METHODS: The first principles based on the density functional theory and density functional perturbation theory were employed to execute all calculations by using the CASTEP code in this work.
RESUMO
Currently, only one crystal structure of LLM-105 (2,6-diamino-3,5-dinitropyrazine-1-oxide) (P21/n) has been discovered, and there are still debates on its phase transition point and phase diagram. Based on previous work, we performed crystal structure, Raman spectra, and vibrational properties calculations on LLM-105 crystal. Our results indicate that the crystal structure of LLM-105 remains stable until compressed to 49 GPa, beyond which it may undergo two phase transitions at pressure intervals of 49.0-49.1 GPa and 51.4-51.5 GPa, respectively. Analysis of Raman shift results suggests that these two phase transitions may be reversible, with an intermediate phase possibly acting as a transition phase. Additionally, based on the quasi-harmonic approximation, we fitted the experimental data of LLM-105 lattice expansion state, obtaining the volume at zero pressure and using it for Raman spectra calculations. The results demonstrated the accuracy of this quasi-harmonic approximation method in describing the redshift of Raman peaks during the heating process and the excitation ratio of Raman peaks in different wavenumber ranges.
