Multicationic Tetrahedra Networks: Alkaline-Earth-Centered Polyhedra and Non-Condensed AlN6-Octahedra in the Imidonitridophosphates AE2AlP8N15(NH) (AE=Ca, Sr, Ba).

Abstract

A series of isostructural imidonitridophosphates AE2AlP8N15(NH) (AE=Ca, Sr, Ba) was synthesized at high-pressure/high-temperature conditions (1400 °C and 5-9GPa) from alkaline-earth metal nitrides or azides Ca3N2/Sr(N3)2/Ba(N3)2 and the binary nitrides AlN and P3N5. NH4F served as a hydrogen source and mineralizing agent. The crystal structures were determined by single-crystal X-ray diffraction and feature a three-dimensional network of vertex-sharing PN4-tetrahedra forming diverse-sized rings that are occupied by aluminum and alkaline earth ions. These structures represent another example of nitridophosphate-based networks that simultaneously incorporate AlN6-octahedra and alkaline-earth-centered polyhedra, with aluminum not participating in the tetrahedra network. They differ from previously reported ones by incorporating non-condensed octahedra instead of strongly condensed octahedra units and contribute to the diversity of multicationic nitridophosphate network structures. The results are supported by atomic resolution EDX mapping, solid-state NMR and FTIR measurements. Eu2+-doped samples show strong luminescence with narrow emissions in the range of green to blue under UV excitation, marking another instance of Eu2+-luminescence within imidonitridophosphates.