High-pressure X-ray investigation of the moganite- and quartz-type phases of phosphorus oxynitride

The high-pressure behavior of the moganite-type polymorph of phosphorus oxy nitride PON (monoclinic, I2/a, Z = 12) has been determined by angle dispers ive X-ray powder diffraction in a diamond anvil cell to 50 GPa. The pressur e variations of the three cell parameters display a distinct slope change: the axial compressibilities are about three times larger below 9 GPa than a bove this pressure. This behavior is completely reversible. The molar volum e of the moganite-type phase under pressure remained larger than that of th e quartz-type phase determined under similar conditions. The pressure-induc ed amorphization of the quartz- and moganite-type phases is compared: while it was reversible upon decompression for moganite it was not for the quart z-phase. This difference in behavior is attributed to local configurations in the structures of the two phases. (C) 2000 Elsevier Science Ltd. All rig hts reserved.