Christian Chateau

The use of composition and high pressure to extend the range of α-quartz isotypes

The use of high pressure and the inclusion of elements such as boron, nitrogen and fluorine increase the number of compounds that adopt the α-quartz-type structure. The α-quartz-type forms of boron phosphate (BPO4), phosphorus oxynitride (PON) and representatives of the continuous solid solution between silica and phosphorus oxynitride, including the nearly stoichiometric material SiPO3N, were prepared under high-pressure, high-temperature conditions. The crystal structures of BPO4 and PON were refined using X-ray and neutron powder diffraction data, respectively. The intertetrahedral A-X-A′ bridging angles of 140.1° and 140.6°, respectively, are slightly lower than in α-quartz as would be expected based on the higher densities of these materials. Preliminary refinements using the X-ray data from the SiO2-PON solid solutions provide no indication of Si/P order and yield fractional atomic coordinates that are very similar to those observed for the pure end members. These results indicate that one may continuously tune the unit cell and structural parameters by varying the composition of this solid solution. Other compounds, which adopt or may adopt α-quartz-type structures, are also discussed. The present work indicates that the known relationship between the intertetrahedral A-X-A′ bridging angle and the density in α-quartz-type materials also applies to boron compounds, oxynitrides and fluorides. This use of the variables composition and pressure enables structure-property relationships for α-quartz isotypes to be extended and increases the number of potential candidates for piezoelectric materials.

A moganite-type phase in the silica analog phosphorus oxynitride

Abstract A new polymorph of phosphorus oxynitride (PON) a silica analog has been recovered at ambient pressure by quenching after a treatment at 850 °C under a pressure of 2.5 GPa using cristobalite- or quartz-type phases as starting materials. This PON polymorph is a thermodynamically stable phase with its own stability field in a P-T diagram. The structure of this PON phase was refined by the Rietveld method from an X-ray powder diffractogram. It is isostructural with “moganite” (SiO2). The discovery of this PON polymorph should stimulate a renewed interest in the occurrence of this phase in the silica system, because “moganite” may have a small but defined P-T stability field. These results confirm the structure of “moganite” as a new structure-type in AX2 compounds.

Crystal structure of moganite-type phosphorus oxynitride: relationship to other twinned-quartz-based structures.

The structure of moganite-type phosphorus oxynitride quenched from high-pressure high-temperature conditions has been refined using neutron powder diffraction data. This moganite-type structure, space group I2/a, Z = 12, is slightly less distorted with respect to the Imab aristotype than is moganite (a monoclinic form of silica). A close topological relationship has been identified between the moganite-type and orthorhombic BeH(2) structures indicating that SiO(2), PON and BeH(2) all adopt structures belonging to the twinned-quartz-based group. This group represents another possible structure type for systems composed of corner-sharing AX(4) tetrahedra. Structures of this group are obvious candidates for intermediate phases between the cristobalite and quartz types.

Crystal structures of α-quartz homeotypes boron phosphate and boron arsenate: structure-property relationships

Abstract The structures of single crystals of the α-quartz homeotypes boron phosphate and boron arsenate prepared under high-pressure, high-temperature conditions in a belt-type apparatus were determined by X-ray diffraction. Both structures are more distorted with respect to the β-quartz structure type than α-quartz itself with average tetrahedral tilt angles δ of 19.7° and 24.6° for BPO4 and BAsO4, respectively. The structure of BAsO4 is one of the most highly distorted found among α-quartz homeotypes. It is shown that for the known α-quartz homeotypes, the density, which can be related to the frequency constant for piezoelectric resonators, scales with the intertetrahedral bridging angle θ.

High pressure behaviour of the -cristobalite-type phase of phosphorus oxynitride, PON

Phosphorus oxynitride, PON, is isoelectronic with and can be used as a model for silica. The crystalline behaviour of the -cristobalite-type phase was determined as a function of pressure by angular dispersive x-ray diffraction in a diamond anvil cell to 48 GPa. No evidence for a phase transition or amorphization was observed. The cell parameters were measured as a function of pressure. The macroscopic bulk modulus is 71(3) GPa with a first pressure derivative of 3.2(5). The polyhedral bulk modulus was inferred to be about 800 GPa, assuming that the tetrahedra remain regular.

The high-pressure behaviour of the “moganite” polymorph of SiO2

The high-pressure behaviour of the “moganite” polymorph of SiO 2 has been investigated by angle-dispersive, X-ray powder diffraction in a diamond anvil cell at room temperature. The bulk modulus was calculated from the P-V data: B 0 = 32.2(3) GPa with B’ 0 fixed to 5.2 as for quartz. The molar volume of moganite becomes lower than the volume of quartz above 5 GPa. Pressure-induced amorphisation occurred progressively; it was complete above 25 GPa. Upon decompression, amorphisation was partially reversible if the maximum pressure reached was below 25–30 GPa and irreversible when the pressure reached was above 45 GPa. The pressure-induced amorphisation of the quartz and moganite polymorphs of silica and phosphorus oxynitride PON, a silica analogue, is related to the distortion of the tetrahedra and, in the case of silica, to the presence of a much denser phase, in which the cation has a higher coordination number.

Luminescence of the Europium(III) Doped High Temperature and Pressure Modification of Rare Earth Disilicates

The UV and dye laser excited optical luminescence spectra at 77 and 300 are reported for the D-Y2Si207:Eu3+ and D-Lu2Si207:Eu3+ systems obtained at high temperature and pressure. The 7/O-s energy level schemes derived from this study are analyzed in terms of a 14 parameter crystal field (cf.) Hamiltonian in a cf. of C2 symmetry. The parameter sets are used to simulate the experimental energy level schemes. The best fit sets of the cf. parameters yield excellent simulation of the experimental cf. splittings of the Fa

Parametric analysis of crystal-field effects in rare-earth-metal disilicates doped with trivalent europium

The u.v. and dye-laser excited luminescence spectra of the Eu3+-doped rare-earth-metal disilicates C-M2Si2O7(M = Y, Lu, Sc, or In) were recorded at 77 and 300 K. The 7F0–5 energy-level schemes of the Eu3+ ion derived from the spectral analysis were simulated with the aid of phenomenological crystal-field theory. The C2 symmetry simulation with nine real and five imaginary parameters of the experimental crystal-field splittings of the energy levels was excellent with root mean square deviations varying between 4 and 6 cm–1. The best-fit parameter values behave in a smooth manner as a function of the host cation in the structurally isomorphic disilicate series. The indium disilicate has slightly different values, however. The strength of the crystal-field effect decreases slightly with increasing ionic radius of the rare-earth-metal host. A comparison of the experimental parameter sets with those obtained for Eu3+-doped rare-earth-metal oxysalts revealed fundamental differences.

Optical properties of Eu3+-doped tetragonal lutetium oxyhydroxide

Abstract The UV and dye-laser-excited luminescence spectra of the Eu 3+ -doped tetragonal high-pressure form of LuOOH were measured at 77 and 300 K. The analysis of the emission spectra yielded a complete crystal-field (c.f.) energy-level scheme for the 7 F 0–4 levels. The simulation of c.f. splittings was conducted in the 25 Stark-level basis set in C 2 v and C 2 symmetries. A satisfactory simulation was obtained only with the C 2 c.f. parameter set. A comparison with the results of a c.f. analysis of the monoclinic form of LuOOH was also carried out.

A.c. susceptibility, microstructure and microchemistry of RBa2Cu3O7−x superconducting ceramics (R Y, Ho)☆

Abstract A study of different kinds of RBa2Cu3O7−x (R  Y and Ho) high T c superconducting ceramics has been carried out in order to correlate physical properties and microstructure. The present paper emphasizes upon the relation between the magnetic behaviour and the microstructure for two samples. The first one corresponds to Ho1Ba2Cu3O7−x treated with a classical sintering and the second to Y1Ba2Cu3O7−x heat treated under high pressure. The microstructure has been characterized by analytical transmission electron microscopy. The magnetic behaviours have been investigated by the susceptibility measurements using an alternative magnetometer. The strong difference between the magnetic behaviours for both samples is the single step or double step penetration of the field. This can be correlated with the microstructures. The first sample presents unhomogeneous chemical compositions and a large amount of extended defects gives only one step. The second sample which is chemically homogeneous and present a small amount of extended defects give a well-defined difference between the intergrain and intragrain material.