The new phase Mg7Pt4Ge4(≡Mg8□1Pt4Ge4; □
= vacancy) was prepared by reacting a mixture of the corresponding
elements at high temperatures. According to single crystal X-ray diffraction
data, it adopts a defect variant of the lighter analogue Mg2PtSi (≡Mg8Pt4Si4), reported
in the Li2CuAs structure. An ordering of the Mg vacancies
results in a stoichiometric phase, Mg7Pt4Ge4. However, the high content of Mg vacancies results in a violation
of the 18-valence electron rule, which appears to hold for Mg2PtSi. First principle density functional theory calculations
on a hypothetical, vacancy-free “Mg2PtGe”
reveal potential electronic instabilities atEFin the band structure and significant occupancy of states
with an antibonding character resulting from unfavorable Pt–Ge
interactions. These antibonding interactions can be eliminated through
introduction of Mg defects, which reduce the valence electron count,
leaving the antibonding states empty. Mg itself does not participate
in these interactions. Instead, the Mg contribution to the overall
bonding comes from electron back-donation from the (Pt, Ge) anionic
network to Mg cations. These findings may help to understand how the
interplay of structural and electronic factors leads to the “hydrogen
pump effect” observed in the closely related Mg3Pt, for which the electronic band structure shows a significant amount
of unoccupied bonding states, indicating an electron deficient system.