Phosphorus oxynitride PON, a silica analogue: structure and compression ofthe cristobalite-like phase; P-T phase diagram

The structure of the cristobalite-like polymorph of phosphorus oxynitride P ON has been refined using neutron powder diffraction data. It is tetragonal , space group I (4) over bar 2d, Z = 4. The four P-(O,N) distances are equa l but the tetrahedron is compressed along c. In AX(2) or ABX(4) compounds, the tetragonal I (4) over bar 2d or I (4) over bar structure is obtained wh en the average ratio of the cation to anion radius is below 1.186, whereas the tetragonal P4(1)2(1)2 or orthorhombic C222(1) structure is obtained at low temperatures for larger ratios. The cell parameters of this PON polymor ph have been determined as a function of hydrostatic pressure by in situ an gle dispersive X-ray powder diffraction in a diamond-anvil cell. Under trul y hydrostatic pressure, a strong anisotropic behavior is observed with the c parameter being nearly incompressible. Very slight anisotropic stress str ongly modifies the high-pressure behavior. According to the pressure-temper ature conditions of treatment, three phases, cristobalite-, moganite-, and quartz-like, have been obtained by quenching experiments, and the P-T phase diagram of PON was derived. The high-pressure behavior or the alpha -quart z, moganite, and cristobalite-like polymorphs of PON and SiO2 is discussed.