Structural and electronic properties of clathrate-like hydride: MH₆ and MH₉ (M = Sc, Y, La)

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 Im3¯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.