Figure 2.

The structures of tetragonal LP Na₃FeH₇ (left) and orthorhombic LP Na₃CoH₆ (right) are shown. (a) P4₂/mnm structure of LP Na₃FeH₇ and Pnma structure of LP Na₃CoH₆ according to DFT optimization. Na ions are depicted as golden circles, and complexes of (TM)H_n^3– are depicted as green polyhedra. (b) Local coordination of (TM)H_n^3– complexes by 8 Na cations (distorted cube) and structure of (TM)H_n^3– complexes. Na, Ni, and H atoms are depicted as golden, green, and gray circles, respectively. Interatomic TM–H distances are indicated (in Å). (c) Arrangement of distorted Na₈ cubes (blue polyhedra) in the LP structures. The central building unit is a pair of edge-condensed distorted cubes (highlighted with a darker color). (d) Z = 32 MD snapshot of HT-HP Na₃FeH₇ and Na₃CoH₆ at 1000 K after equilibration. (TM)H_n^3– complexes are drawn as green polyhedra.