Introduction to Zintl phases
Zintl phases are valence-balanced semiconductors, where typically electropositive group 1 or 2 elements donate electrons to more electronegative group 13-16 elements.
For example, in NaSi (Na4Si4, see Figure 1), the electronic situation can be described as follows:
- Each Na atom donates 1e–.
- Each Si atom accepts 1e–
- [Si4]4– tetrahedra are isoelectronic with P4 tetrahedra (white phosphorus)
As a result, there is covalent bonding within the Si tetrahedra and ionic bonding between the anionic polyhedra and the cations. Basically, the electron transfer in this case follows the well-known 8-electron rule. Ideally, Zintl phases should be semiconductors.
Figure 1. Na4Si4 (C2/c) with [Si4]4– tetrahedra (blue) and Na+ cations (violet).