A high-pressure polymorph of phosphorus oxonitride with the coesite structure.

The chemical and physical properties of phosphorus oxonitride (PON) closely resemble those of silica, to which it is isosteric. A new high-pressure phase of PON is reported herein. This polymorph, synthesized by using the multianvil technique, crystallizes in the coesite structure. This represents the first occurrence of this very dense network structure outside of SiO2 . Phase-pure coesite PON (coe-PON) can be synthesized in bulk at pressures above 15 GPa. This compound was thoroughly characterized by means of powder X-ray diffraction, DFT calculations, and FTIR and MAS NMR spectroscopy, as well as temperature-dependent diffraction. These results represent a major step towards the exploration of the phase diagram of PON at very high pressures and the possibly synthesis of a stishovite-type PON containing hexacoordinate phosphorus.