Structures of phosphonitrides in light of the extended Zintl-Klemm concept

The extended Zintl-Klemm concept (EZKC) is applied to explain the crystalline structures of phosphonitrides (also known as nitridophosphates in the chemical literature). The examples of (AE)₂AlP₈N₁₅(NH) (AE = Ca, Sr, Ba), Ge^IVPN₃ and MP₂N₄ (M = Be, Ca, Sr, Ba, Ge^II) are mainly discussed, although the examples of LiGaGe and LiGaGeO₄ have been also commented on due to their relation with BeP₂N₄. It is shown that the EZKC provides a better understanding of the structures of these compounds than in previous descriptions. In most of these nitrides, P atoms behave as pseudo-Si atoms and N atoms behave as pseudo-O atoms, so providing a good explanation for the four-connectivity of P atoms forming PN₄ units, which behave as pseudo-SiO₄ units like the SiO₄ units in many polymorphs of SiO₂. In addition, the EZKC shows that the notation of these compounds as phosphonitrides is more appropriate than as nitridophosphates because N atoms act as the anions in these compounds.