Manganese hydrides and superhydrides at high pressure

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 ${\mathrm{Mn}}₂\mathrm{H}$ (space group $P\overline{3}m1$) is the only compound found stable among those studied, since $\ensuremath{\epsilon}$-MnH (hcp structure, space group $P{6}₃/mmc$), although stable at 0 K, is destabilized by thermal effects. With increasing pressure, MnH is stabilized, and then hydrogen-rich hydrides appear: first ${\mathrm{MnH}}₂$ around 30 GPa, then ${\mathrm{MnH}}₇$ around 40 GPa, and finally ${\mathrm{MnH}}₃$ around 90 GPa. ${\mathrm{MnH}}₈$ is likely to become stable around 150 GPa. The body-centered tetragonal structure of ${\mathrm{MnH}}₂$ (space group $I4/mmm$) is identical to that of ${\mathrm{FeH}}₂$ and ${\mathrm{CoH}}₂$. ${\mathrm{MnH}}₃$ has an original body-centered tetragonal structure, with space group $I4/m$ and 4 formula units per primitive cell. It contains a structural unit ${\mathrm{H}}₈$ with rather short H-H distances. ${\mathrm{MnH}}₇$ is extremely stable at high pressure. ${\mathrm{MnH}}₇$ and ${\mathrm{MnH}}₈$ exhibit a mixed structure, with one hydrogen being under the molecular form (1/2 ${\mathrm{H}}₂$) 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 ${\mathrm{MnH}}₃$, and then saturating beyond. At high pressure, nuclear quantum effects tend to stabilize ${\mathrm{MnH}}₄$ and increase the stability of ${\mathrm{MnH}}₈$.