A. Mosset

Sol-gel processed BaTiO3 : structural evolution from the gel to the crystalline powder

An amorphous solid, precursor to BaTiO3, has been prepared by the sol-gel route and its structural organization studied by LAXS as a function of temperature. In the low-temperature solid, the barium atoms keep the same environment as in barium acetate. The titanium alkoxide reacts with acetic acid to give an hexanuclear molecule.

Synthesis and structural characterization of non-crystalline mullite precursors

Abstract Two different types of non-crystalline mullite precursor with identical bulk composition (72 wt% Al 2 O 3 , 28 wt% SiO 2 ) were prepared from tetraethoxysilane and silicon chloride, respectively, and aluminium sec-butozide, by using different methods of hydrolysis. The precursors, designated as type I and III, display different crystallization processes above ≈ 900°C: type I precursors directly form mullite, while type III precursors yield crystallization of transient γ-alumina. Infrared (IR) spectroscopy, large angle X-ray scattering (LAXS) and 27 Al nuclear magnetic resonance spectroscopic studies, and 29 Si nuclear magnetic resonance (NMR) literature data give evidence for a high degree of structural mixing in type I precursors and for a beginning of segregation into Al 2 O 3 -rich domains in type III precursors prior to crystallization (⩽ 900°C). Both precursors are composed of (SiO) tetrahedra and of (AlO) octahedra, tetrahedra and pentahedra although pentahedra are dominant in type I while octahedra occur more frequently in type III precursors. The driving force for mullitization (type I) and γ-alumina formation (type III) taking place at the same temperature is believed to be the instability of pentahedrally coordinated Al above ≈ 900°C. The sudden disappearance of Al pentahedra probably depends on the formation of reactive network centers during dehydroxylation. This hypothesis is derived from the observation that dehydroxylation and condensation strongly take place in a similar temperature range prior to crystallization.

Structural analysis of amorphous V2O5 by large-angle X-ray scattering

Abstract An amorphous thin plate of vanadium pentoxide, V2O5, prepared by ultrafast quenching from the melt (splat-cooling) has been examined by large-angle X-ray scattering (LAXS) and its structure thereby investigated. The intensities at 450 points within the θ range 2° < θ < 70° were measured at room temperature (22°C). The oxygen coordination around the vanadium atoms is tetrahedral, with an average V—O distance of 1·75 A. The proposed structure is based on sub-units of three VO4 tetrahedra sharing corners, the relative orientation of which has been determined. The V2O5 formula is retained by following the rule that each tetrahedron shares three corners with other VO4 tetrahedra. Experimental and theoretical intensity and radial distribution curves have been compared to support the model. A proposal is made concerning the transition of the crystal structure of V2O5 to the molten and then amorphous states phases, which are supposed to be strongly structurally correlated.

Effect of some dopant elements on the low temperature formation of γ-Ce2S3

Abstract For the first time, it has been clearly shown that carbon is an excellent element to stabilize the γ-phase of cerium sesquisulfide at a low temperature (around 800°C) by using CS 2 . The most appropriate route is to use cerium molecular-complex precursors which contain some carbon (like acetate, succinate…), but this element can also be added to carbon-free precursors, such as nitrate. However, the γ-phases so-obtained are dark or brown due to the presence of graphitic carbon. Silicon presents the same properties as carbon. Likewise, the use of phosphate precursors leads to the γ-phase at low temperature (750–800°C). However, during the thermal treatment under CS 2 , some phosphorus is lost from the solid-phase resulting in a progressive transformation of the γ-phase into the α-phase. Contrary to the α-Ce 2 S 3 phase, the residual monazite (CePO 4 ) does not greatly affect the red color of γ-Ce 2 S 3 . The same results are observed by using borate and, to a lesser extent, vanadate and arseniate precursors. One can assume that the stabilization of the γ-phase is probably due to the insertion of a small amount of these dopants into the empty tetrahedral cavities of the γ-Ce 2 S 3 structure.

The stabilization of γ-Ce2S3 at low temperature by heavy rare earths

Abstract The use of some heavy lanthanide(III) elements allows the stabilization of the γ-phase of Ce 2 S 3 synthesized in H 2 S at low temperatures (600°C–800°C). Among the lanthanides used, the best element is dysprosium followed by holmium, erbium and terbium. The stabilization is more pronounced when using mixed-precursors, either the crystallized oxalate or other less crystallized complexes. However this stabilization is only transitory: heating above 800°C yields the β form. It is the first time that the γ phase is observed at a lower temperature than the β form. Due to the low temperatures and the short treatment time no change of morphology is observed between the starting precursor and the final sulphide.

XRD and 29Si MAS-NMR study on some non-crystalline silica minerals

Abstract Opal-AG, opal-AN (hyalite) and natural silica glass (Libyan desert glass) have been investigated by chemical analyses, near infrared spectroscopy, thermal analyses (DTA and TGA), large angle X-ray scattering and solid-state magic angle spinning NMR spectroscopy. Non-crystalline opals (opal-AG and opal-AN) have a different tetrahedral network than silica glass with larger mean Siue5f8Oue5f8Si angles and a comparatively narrow distribution of Siue5f8Oue5f8Si angles. The difference indicate that non-crystalline opals are almost exclusively built up of interconnected six-membered rings of [SiO4]-tetrahedra while silica glass also contains other structural elements including odd membered rings of [SiO4]-tetrahedra.

Study of some ternary and quaternary systems based on γ-Ce2S3 using oxalate complexes: stabilization and coloration

Abstract In the ternary system, Ce–Na–S, starting with oxalate precursors and using H2S as sulfurizing agent, the sodium doped γ-Ce2S3 becomes the main phase for Na/Ce=0.1 and is pure for Na/Ce=0.2 at 700°C. The addition of dysprosium (oxalate) to this system allows the stabilization of the γ-phase as long as the atomic ratios take the values Dy/Ce≤2 and Na/[Ce+Dy]≥0.1, and the temperature of treatment is equal or higher than 900°C. The secondary phases are the oxysulfide Ce2−xDyxO2S and the mixed sulfide NaDyS2. By substituting erbium for dysprosium the amount of secondary phases increases. To improve the coloration of the sulfide, the oxalate precursor must undergo a thermal treatment in air, before its sulfurization, in order to decrease the carbon amount of the sulfides. The addition of lanthanide(III) to the ternary system modifies slightly the color of the resulting sulfide.

Sol-gel processing and structural study of a vitreous polysiloxane doped with titanium

Monolithic crack-free condensates have been synthesized by hydrolysis and polycondensation of tetraethoxytitanium(IV) and γ-glycidoxypropyltrimethoxysilane. The local order in these materials has been studied by large angle X-ray scattering (LAXS) as well as the influence of Ti4+-Ti3+ transition induced by UV irradiation. The polysiloxane is characterized by a quasiregular arrangement of the side-chains similar to the proposed models for liquid-crystalline polysiloxane. Titanium atoms, in an octahedral environment, are coordinated to the diol functions. If the metal percentage is higher than 20%, the titanium atoms are distributed on several sites.

New potential open-framework materials in gallophosphate systems. Preparation and crystal structure of [NH3(CH2)3NH3]Ga3(PO4)3(OH)2·H2O and [NH2(CH2)3NH2]Ga3(PO4)3(H2O)

The synthesis and the structure of two new gallium phosphates [NH3(CH2)3NH3]Ga3(PO4)3(OH)2·H2O (GaP1) space group Pcab, a = 10.043(3), b = 15.989(4), c = 18.308(6)xa0A and [NH2(CH2)3NH2]Ga3(PO4)3(H2O) (GaP2) space group Pbc21, a = 8.903(3), b = 9.697(3), c = 16.326(7)xa0A are presented. The syntheses were realised under hydrothermal conditions at 200u2006°C in the system Ga2O3–H3PO4–1,3-diaminopropane–H2O. These two compounds crystallise with a 3D-framework, with localised channels. In the GaP2 structure, 1,3-diaminopropane (1,3-DAP) is, unusually, coordinated to a gallium atom.

Structural studies of amorphous Zn-P films

Large angle X-ray diffraction studies were performed on vapour deposited Zn-P amorphous films with phosphorus contents of 43, 50 and 68 at %. Using MoKα radiation, measurements have been made for 4πsinθ/λ between 0.06 and 1.6 nm−1. The experimental method and data reduction procedure are described. The structure of the amorphous Zn-P films is analysed using both intensity and radial distribution functions. The experimental results are compared with a number of paracrystalline models based on various crystalline polymorphs of the II-V system. Comparison shows that reasonable agreement is achieved for distorted tetrahedral structures: the intermediate between CdAs and Si type III for Zn57P43 and Zn50P50 and the distorted CdAs2 for Zn32P68. The sizes of the microparacrystals are evaluated from the integral widths of the first diffraction peaks and the lattice distortions by applying paracrystalline theory. It is shown that the size of the microparacrystal and the degree of the lattice distortion are correlated by the α* relation for the investigated Zn-P amorphous films.