Crystal structure and high pressure behaviour of the quartz-type phase of phosphorus oxynitride PON

Phosphorus oxynitride PON is isoelectronic with SiO2 and both exhibit crist obalite- and quartz-type phases. The alpha-quartz phase of PON was prepared by submitting the beta-cristobalite phase to 4.5 GPa and 750 degrees C in a belt type apparatus. The structure at ambient was refined using time-of-f light neutron powder diffraction data on a quenched sample. The structure ( trigonal, P3(2)21, a = 4.757(4), c = 5.2460(2) Angstrom, Z = 3) is essentia lly the same as that of alpha-quartz SiO2. The atomic positions and tempera ture factors of phosphorus, oxygen and nitrogen were obtained. Oxygen and n itrogen are completely disordered over the 6(c) sites. The behaviour of alp ha-quartz-type PON under pressure was determined by angle dispersive X-ray powder diffraction in a diamond anvil cell up to 48 GPa. The cell parameter s were measured as a function of pressure. A transition to an alpha-quartz II phase occurs at close to 20 GPa; the main change is a small, but distinc t increase in the c/a ratio. Progressive and irreversible amorphisation of PON was observed above 30 GPa, and no diffraction lines could be detected a bove 42 GPa. For both PON and SiO2, the alpha-quartz-type phases undergo ph ase changes under pressure to quartz II phases followed by amorphisation. T his pressure study reinforces the parallel behaviour of the two compounds. (C) 1998 Elsevier Science Ltd. All rights reserved.