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

Abstract

Phosphorus oxynitride PON is isoelectronic with SiO 2 and both exhibit cristobalite- and quartz-type phases. The α-quartz phase of PON was prepared by submitting the β-cristobalite phase to 4.5 GPa and 750°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 221, a=4.757(4), c=5.2460(2) Å, Z=3) is essentially the same as that of α-quartz SiO 2. 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 α-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 α-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 SiO 2, the α-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.