Zintl phases with delocalized bonding

The La-Si binary system is very attractive in relation with elemental carbon chemistry. LaSi2 adopts the a-ThSi2 structure as shown in Fig. 7.8d and the coordination of Si in LaSi2 is a planner triangle. The structure is composed of sp -bonded silicon. LaSi2 is metallic and shows superconductivity with Tc = 2.5 K [55]. LaSi2 is a Zintl phase with delocalized electrons. If this sub-network is realized with all- p carbon without metal atoms, a new 3D carbon aUotrope will be obtained. The hypothetical 3D all-jp carbon phase is called hyper graphite [56, 57]. Metallic conduction is expected from theoretical band stracture calculations of the hypothetical structure. 

Polar intermetallics are loosely referred to as electron-poorer relatives of Zintl phases in which the active metals do not contribute all of their valence electrons, rather they bond with the more electronegative components to some degree. The structures cannot be simply accounted for by octet rules because of substantial delocalized bonding among the atoms. 

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