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

The structure of the cristobalite-like poly- morph of phosphorus oxynitride PON has been re®ned using neutron powder di?raction data. It is tetragonal, space group I?42d, Z ? 4. The four P±?O,N) distances are equal but the tetrahedron is compressed along c. In AX2 or ABX4 compounds, the tetragonal I?42d or I?4 structure is obtained when the average ratio of the cation to anion radius is below 1.186, whereas the tetragonal P41212 or orthorhombic C2221 structure is obtained at low tem- peratures for larger ratios. The cell parameters of this PON polymorph have been determined as a function of hydrostatic pressure by in situ angle dispersive X-ray powder di?raction in a diamond-anvil cell. Under truly hydrostatic pressure, a strong anisotropic behavior is observed with the c parameter being nearly incom- pressible. Very slight anisotropic stress strongly modi®es the high-pressure behavior. According to the pressure-J. M. Le ger ?8) á C. Chateau á G. Bocquillon CNRS, Laboratoire des Proprie te s Me caniques et Thermodynamiques des mate riaux,1, Place Aristide Briand,92190 Meudon, FranceTel.: +33-0-145075033; Fax: +33-0-1455910 e-mail: leger@CNRS-bellevue.frJ. HainesLaboratoire de Physico-Chimie de la MatieÁ re Condense e, UMR 5617 ± CNRS, Universite Montpellier II, cc003, 34095 Montpellier ce dex 5, FranceM. W. SchmidtCNRS-UMR6524, Magmas et volcans, 5 rue Kessler, 63038 Clermont-Ferrand ce dex, FranceS. HullISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, UKF. GorelliEuropean Laboratory for Non-linear Spectroscopy, Largo Enrico Fermi 2, 50125 Florence, ItalyA. Lesauze á R. MarchandLaboratoire Verres et Ce ramiques, UMR CNRS 6512, Universite de Rennes I, Campus de Beaulieu,35042 Rennes, Francetemperature conditions of treatment, three phases, cris- tobalite-, moganite-, and quartz-like, have been obtained by quenching experiments, and the P±T phase diagram of PON was derived. The high-pressure behavior of the a-quartz, moganite, and cristobalite-like polymorphs of PON and SiO2 is discussed.