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 cristobalite- 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-flight 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 essentially the same as that of alpha-quartz SiO2. The atomic positions and temperature factors of phosphorus, oxygen and nitrogen were obtained. Oxygen and nitrogen are completely disordered over the 6(c) sites. The behaviour of alpha-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 parameters 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 distinct increase in the c/a ratio. Progressive and irreversible amorphisation of PON was observed above 30 GPa, and no diffraction lines could be detected above 42 GPa. For both PON and SiO2, the alpha-quartz-type phases undergo phase changes under pressure to quartz II phases followed by amorphisation. This pressure study reinforces the parallel behaviour of the two compounds. (C) 1998 Elsevier Science Ltd. All rights reserved.