Origin of the 20-electron structure ofMg3MnH7: Density functional calculations

Abstract

The electronic structure and stability of the 20-electron complex hydride, ${\mathrm{Mg}}_{3}{\mathrm{MnH}}_{7}$ is studied using density functional calculations. The heat of formation is larger in magnitude than that of ${\mathrm{MgH}}_{2}$. The deviation from the 18-electron rule is explained by the predominantly ionic character of the band structure and a large crystal-field splitting of the Mn $d$ bands. In particular, each H provides one deep band accomodating two electrons, while the Mn ${t}_{2g}$ bands hold an additional six electrons per formula unit.