Compression of PON cristobalite to 70GPa

We have compressed phosphorus oxynitride (PON) in a diamond-anvil cell to 70 GPa. The PON has a distorted cristobalite structure and is isoelectronic with SiO2. In situ micro-Raman spectroscopy and synchrotron X ray diffraction show that PON cristobalite undergoes a phase transformation in the range of 12 to 20 GPa. Based on the similarity of X ray and Raman spectra before and after the transition, the high-pressure phase also has a cristobalite-type structure. Remarkably, this open-framework structure remains stable to the maximum experimental pressures. Although there is evidence for a reversal transformation of the high-pressure phase near 17 GPa upon decompression, the crystalline phase quenched to ambient conditions is not identical to the starting material. No amorphization of crystalline PON is observed during the in situ compression and decompression measurements. However, X ray investigation of the decompressed sample at ambient pressure reveals a mixture of amorphous and crystalline PON components.