Optimization of Chemical Bonding through Defect Formation and Ordering─The Case of Mg 7 Pt 4 Ge 4 .

The new phase Mg 7 Pt 4 Ge 4 (≡Mg 8 □ 1 Pt 4 Ge 4 ; □ = 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 Mg 2 PtSi (≡Mg 8 Pt 4 Si 4 ), reported in the Li 2 CuAs structure. An ordering of the Mg vacancies results in a stoichiometric phase, Mg 7 Pt 4 Ge 4 . However, the high content of Mg vacancies results in a violation of the 18-valence electron rule, which appears to hold for Mg 2 PtSi. First principle density functional theory calculations on a hypothetical, vacancy-free "Mg 2 PtGe" reveal potential electronic instabilities at E F in 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 Mg 3 Pt, for which the electronic band structure shows a significant amount of unoccupied bonding states, indicating an electron deficient system.