Structural features of the oxidonitridophosphates K₃ M ^III(PO₃)₃N (M ^III = Al, Ga)

Cubic crystals of tripotassium aluminium (or gallium) nitridotriphosphate, K₃ M ^III(PO₃)₃N (M ^III = Al, Ga), were grown by application of the self-flux method. In their isostructural crystal structures, all metal cations and the N atom occupy special positions with site symmetry 3, while the P and O atoms are situated in general positions. The three-dimensional framework of these oxidonitridophosphates is built up from [M ^IIIO₆] octa-hedra linked together via (PO₃)₃N groups. The latter are formed from three PO₃N tetra-hedra sharing a common N atom. The coordination environments of the three potassium cations are represented by two types of polyhedra, viz. KO₉ for one and KO₉N for the other two cations. An unusual tetra-dentate type of coordination for the latter potassium cations by the (PO₃)₃N^6- anion is observed. These K₃ M ^III(PO₃)₃N (M ^III = Al, Ga) compounds are isostructural with the Na₃ M ^III(PO₃)₃N (M ^III = Al, V, Ti) compounds.