Nicole Floquet

Ferroelectric domain walls in BaTiO3: Structural wall model interpreting fingerprints in XRPD diagrams

Abstract Accurate analyses of X-ray diffraction patterns of micrometric BaTiO3 grains has shown that 90° domain wall is not a single twin (101) plane but a few nanometers wide region where the twinning crystallographic discontinuity could be accommodated by atom plane translations. A structural model of the 90 ° domain wall based on curved (001) and (100) planes is proposed. Its theoretical diffraction treatment is developed. The good agreement between experimental fingerprints and theoretical line profiles allows to propose a wall thickness of around 140 A for micrometric BaTiO3 grains whereas their domain thickness is of the order of 0.1μm.

Growth of large Mo18O52 single crystals by a vapor phase method

Abstract Mo 18 O 52 single crystals are obtained by a gas phase method in sealed silica tubes. Compared with the known chemical vapor transport method, this crystallization is effected at uniform and constant temperature, in presence of iodine gas. Various crystallization parameters are studied. A set of experimental conditions are determined to obtain large single crystals.

Water sorption in hydrophobic porous materials : isotherm shapes and their meanings for the mesoporous MCM-41 and the microporous AIPO4-5

We report on extensive neutron diffraction and incoherent quasi-elastic neutron scattering analyses for the water sorption in two hydrophobic porous materials: the mesoporous material MCM-41 and the microporous zeolite AIPO 4 -5. Water sorption isotherms have, in the both porous materials, the characteristics of type V isotherms: vertical step at p/p 0 > 0.3 and Hl hysteresis loop. Whatever the pore diameter (either mesoporous 20 A < 0 < 40 A or microporous 0 =7.3 A), whatever the pore wall structure (either amorphous SiO 2 , or crystalline AIPO 4 ), water sorption phenomenon looks like the so-called capillary condensation phase transition. Our neutron scattering results clearly validate such an expected behaviour in the mesoporous confinement range (20 A < O MCM-41 < 40 A). Concerning water confinement in the microporous range (O AIPO4-5 = 7.3 A), our results are more surprising. Type V sorption isotherm is the signature of a crystallization phenomenon at room temperature (T = 300 K). The confined water crystallizes in two helices that are commensurate with the AIPO 4 -5 micropore structure. The confined ice has a density of 1.2 g.cm -3 .

Adsorption and neutron scattering studies: a reliable way to characterize both the mesoporous MCM-41 and the filling mode of the adsorbed species

Numerous studies concern the MCM-41 material, and yet the MCM-41 porosity description and especially the MCM-41 pore diameter (O) characterization is still open for discussion. Here, we report on adsorption and extensive neutron diffraction analyses for the hydrogen sorption in the mesoporous material MCM-41. The type IV isotherm for hydrogen sorption in MCM-41 is similar to many species sorption isotherms such as N 2 , Ar, Kr. The hydrogen isotherm particularity is that the two distinctive parts of the isotherm (first vertical uptake at the low relative pressure p/p 0 ≤ 0.1 and second uptake at the high relative pressure p/p 0 > 0.1) are equally developed. Thus, the neutron scattering experiments realized during the hydrogen adsorption gave accurate data for each adsorption mechanism associated to these two main uptakes, and consequently for MCM-41 porosity structural characterisation. The main findings are the low density of the MCM-41 silica walls (20% voids) and the fine description of D 2 adsorption mechanism: at the low relative adsorption pressure D 2 fills the wall voids and forms a layer on the rough wall surface. Then at the relative pressure of the second uptake, D 2 fills the whole free MCM-41 mesopores as expected (capillary condensation phenomenon). Even the solid capillary phase does not grow layer by layer on the inner pore walls but is growing up along the pore axis.

Onset of oxidation of the non-stoichiometric molybdenum oxide Mo18O52(100) surface at low temperatures: A study by RHEED

Mo18O52(100) surface oxidation was investigated at constant temperature (670 K) under an oxygen pressure of about 3 × 103 Pa for increasing time up to a few tens of hours. Three oxidation stages were evidenced by RHEED analysis and interpreted by associating RHEED results and SEM observations as a three steps oxidation mechanism: the first step concerns chemisorption of oxygen onto CS plane boundaries, the second one leads to an epitaxial growth of crystallites within the MoO3 structure including randomly spaced CS planes, the third oxidation step proceeds from a sublimation phenomenom and leads to the spread of epitaxial MoO3 films on the whole surface. The proposed oxidation mechanism involves mobility of the CS planes, hyperreactivity of the boundary of the layer structure and also increasing mobility of (MoO3)n species in equilibrium on the oxide surface; these fundamental features of the reaction paths in molybdenum oxides broaden the knowledge of non-stoichiometric oxides of transition metals and are particularly related with the catalytic behaviour of MoO3.

Determination of zeolite closed porosity in (1D) channel systems (AFI and MTW types)

Abstract We have observed large variations in the sorption capacities of zeolite samples characterized by (1D) channel systems, as for instance AFI (AlPO 4 -5 zeolite) and MTW (ZSM-12 zeolite) architectural framework types. Indeed, for such unconnected micropore networks, point defects or chemisorbed impurities can annihilate a huge number of sorption sites. Detailed analysis, by neutron diffraction of the structural properties of the sorbed phase / host zeolite system, has pointed out clear evidence of closed porosity existence. Percentage of such an enclosed porosity has been determined.

Structural and dynamic properties of confined water in model porous materials (AlPO4-5, MCM-41)

Confined water presents unusual properties in comparison with other sorbate species. First of all, the sorption isotherm is of type III, even in the microporous confinement range (O<20 ). Whatever the pore diameter, water sorption phenomenon looks like the so-called capillary condensation phase transition. Our results clearly valid such an expected behaviour in the mesoporous confinement range (20 <O<40 ). The water confined phase is a liquid phase characterized by a short range order and a high translational molecular mobility. The confinement induces a strong displacement towards the low temperature of the water confined liquid solidification Tsol. (for instance, Tsol.=230 K for D2O confined liquid in MCM-41 (O=24 ). We have determined the structure of the water confined solid phase observed below Tsol.. It looks like those of the cubic ice structure affected by strong quasi-isotropic finite size effects induced by the confinement. Such a quasi-(1d) solid appears as a polycrystalline column rather than a single crystalline nano fiber. Concerning water confinement in the microporous range (as for example, AlPO4-5 zeolite (O=7.3 )), our results are more surprising. Type III sorption isotherm is the signature of a crystallization phenomenon at room temperature (T=300 K); A commensurate ice chain characterized by a double helix morphology and a high density (d=1.5) is observed.

Zeolite inner surface characteristics and influence on confined phases

Without any doubt, the zeolite framework porous characteristics (micropores sizes and topology) largely govern the zeolite properties and their industrial applications. Nevertheless for some zeolite uses, as for instance, host materials for confined phases, the zeolite inner surface characteristics should be precised to understand their influence on such low dimensionality sorbed systems. In that paper, we present illustrative examples of zeolite inner surface influence on confined methane phases. Our investigation extends from relatively complex zeolite inner surface types (as for MOR structural types) to the model inner surface ones (well illustrated by the AFI zeolite type). Sorption isotherm measurements associated with neutron diffraction experiments are used in the present study.

Structural properties of confined simple molecules (Ar, CO2, C2D2, CF4 and C(CD3)4) in AlPO4-5 model zeolite

Abstract The present study concerns the structural analysis by neutron diffraction of simple molecular species characterized by van der Waals interactions (Ar, CF 4 , C(CD 3 ) 4 ) and specific interactions (CO 2 , C 2 D 2 ) confined in AlPO 4 -5 model zeolite. In the full loading regime, interesting commensurate quasi-(1D) structures are proposed for CO 2 and C 2 D 2 molecular materials. Incommensurate structures are observed for globular or quasi-globular molecules as Ar, CF 4 and C(CD 3 ) 4 .

Reactivity of the Mo18O52(100) surface: A study by RHEED of the Mo18O52-MoO3 structural transformation

Abstract Large Mo 18 O 52 crystals are obtained by chemical vapor transport in sealed quartz tubes. Their well developed surfaces are the (100) surfaces with steps along the [010] direction as proved by RHEED analysis, and this oxide surface conformation is connected to the particular Mo 18 O 52 structure which is built up of MoO 3 slabs of finite width mutually joined by CS planes. Mo 18 O 52 (100) surface oxidation has been investigated at constant temperature (670 K) under an oxygen pressure of about 3×10 3 Pa for increasing times up to a few tens of hours. Three oxidation stages were indicated by RHEED analysis and interpreted by a combination of RHEED and SEM observations, as three successive steps of oxidation mechanism: the first step is concerned with the chemisorption of oxygen onto CS plane boundaries, the second one leads to an epitaxial growth of crystallites within the MoO 3 structure including randomly spaced CS planes. The third oxidation step proceeds from a sublimation phenomenon and leads to the spread of epitaxial MoO 3 films over the whole surface. The proposed oxidation mechanism involves mobility of the CS planes, hyper-reactivity of the boundary of the structure layer and also increasing mobility of (MoO 3 ) n species in equilibrium on the oxide surface; these fundamental features of the reaction paths in molybdenum oxides broaden the knowledge of nonstoichiometric oxides of transition metals and are related especially to the MoO 3 (0 1 0) surface reactivity and consequently to the MoO 3 catalytic behaviour.