Quaternary and higher oxynitrides

Following ternary phosphorus oxynitrides, which involve only phosphorus as a cationic element, quaternary and higher oxynitrides will be described. A number of other solid-state PON compounds are under study and, when considering the richness of the phosphate crystal chemistry and the additional degree of freedom given by the presence of nitrogen, it is very probable that many new nitridooxophosphate compositions will be found in the near future. 

As for the corresponding iono-covalent ternary oxides, the structure of the quaternary R-M-O-N and higher oxynitrides will be advantageously described fi"om the coordination polyhedra of M atoms, the tetrahedra for example in silicates, the larger R atoms being located within the holes formed by the different arrangements of these structural units. Such a description is particularly well adapted to the case of tetrahedra and octahedra. Three types of environment for M atoms can be pointed out in the rare-earth-containing oxynitrides, namely tetrahedral, octahedral and cubic . 

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