Manganese hydrides and superhydrides at high pressure

摘要

Manganese hydrides and superhydrides have been studied from ambient up to high pressures by the ab initio random structure searching algorithm. At ambient pressure conditions and room temperature, the subhydride Mn2H (space group is the only compound found stable among those studied, since epsilon-MnH (hcp structure, space group P6(3)/mmm), although stable at 0 K, is destabilized by thermal effects. With increasing pressure, MnH is stabilized, and then hydrogen-rich hydrides appear: first MnH2 around 30 GPa, then MnH7 around 40 GPa, and finally MnH3 around 90 GPa. MnH8 is likely to become stable around 150 GPa. The body-centered tetragonal structure of MnH2 (space group 14/mmm) is identical to that of FeH2 and CoH2 center dot MnH3 has an original body-centered tetragonal structure, with space group 14/m and 4 formula units per primitive cell. It contains a structural unit H-8 with rather short H-H distances. MnH7 is extremely stable at high pressure. MnH7 and MnH8 exhibit a mixed structure, with one hydrogen being under the molecular form (1/2 H-2) and the other ones under an atomic (hydrided) form. Atomic charges computed by the Bader method confirm the electron transfer from Mn to H, with Mn oxidation increasing from MnH to MnH3, and then saturating beyond. At high pressure, nuclear quantum effects tend to stabilize MnH4 and increase the stability of MnHg.