Valérie Montouillout

Amorphous materials: Properties, structure, and durability† Structure of Mg- and Mg/Ca aluminosilicate glasses: 27Al NMR and Raman spectroscopy investigations

Abstract The structure and properties of glasses and melts in the MgO-Al2O3-SiO2 (MAS) and CaO-MgOAl2O3- SiO2 (CMAS) systems play an important role in Earth and material sciences. Aluminum has a crucial influence in these systems, and its environment is still questioned. In this paper, we present new results using Raman spectroscopy and 27Al nuclear magnetic resonance on MAS and CMAS glasses. We propose an Al/Si tetrahedral distribution in the glass network in different Qn species for silicon and essentially in Q4 and VAl for aluminum. For the CMAS glasses, an increase of VAl and VIAl is clearly visible as a function of the increase of Mg/Ca ratio in the (Ca,Mg)3Al2Si3O12 (garnet) and (Ca,Mg)AlSi2O8 (anorthite) glass compositions. In the MAS system, the proportion of VAl and VIAl increases with decreasing SiO2 and, similarly with calcium aluminosilicate glasses, the maximum of VAl is located in the center of the ternary system.

71Ga NMR of reference GaIV, GaV, and GaVI compounds by MAS and QPASS, extension of gallium/aluminum NMR parameter correlation.

We report new measurements of NMR parameters for 71Ga in gallium bearing oxide reference compounds, ranging from perfectly ordered systems to disordered crystalline structures and their aluminate counterparts. Static, MAS, and QPASS spectra are obtained at magnetic fields ranging from 7.0 to 18.8 T. With these results we enhance the previously established correlation between isotropic chemical shifts of 71Ga and 27Al and propose a correlation between gallium and aluminum electric field gradients (EFG). This correlation shows that the EFG at 71Ga sites are generally three times greater than those at equivalent 27Al sites.

Order-resolved sideband separation in magic angle spinning NMR of half integer quadrupolar nuclei

Abstract We describe a new pulse sequence that separates spinning sidebands by order for MAS NMR spectra of half integer quadrupolar nuclei broadened to second order. This two dimensional experiment is based on the modulation of each spinning sideband by its order with nine pulses (QPASS: quadrupolar phase adjusted spinning sidebands). The experimental spectra are compared with simulation for the case of 71 Ga in β-Ga 2 O 3 .

Topological, Geometric, and Chemical Order in Materials: Insights from Solid-State NMR

Unlike the long-range order of ideal crystalline structures, local order is an intrinsic characteristic of real materials and often serves as the key to the tuning of their properties and their final applications. Although researchers can easily assess local ordering using two-dimensional imaging techniques with resolution that approaches the atomic level, the diagnosis, description, and qualification of local order in three dimensions is much more challenging. Solid-state nuclear magnetic resonance (NMR) and its panel of continually developing instruments and methods enable the local, atom-selective characterization of structures and assemblies ranging from the atomic to the nanometer length scales. By making use of the indirect J-coupling that distinguishes chemical bonds, researchers can use solid-state NMR to characterize a variety of materials, ranging from crystalline compounds to amorphous or glassy materials. In crystalline compounds showing some disorder, we describe and distinguish the contributions of topology, geometry, and local chemistry in ways that are consistent with X-ray diffraction and computational approaches. We give examples of materials featuring either chemical disorder in a topological order or topological disorder with local chemical order. For glasses, we show that we can separate geometric and chemical contributions to the local order by identifying structural motifs with a viewpoint that extends from the atomic scale up to the nanoscale. As identified by solid state NMR, the local structure of amorphous materials or glasses consists of well-identified structural entities up to at least the nanometer scale. Instead of speaking of disorder, we propose a new description for these structures as a continuous assembly of locally defined structures, an idea that draws on the concept of locally favored structures (LFS) introduced by Tanaka and coworkers. This idea provides a comprehensive picture of amorphous structures based on fluctuations of chemical composition and structure over different length scales. We hope that these local or molecular insights will allow researchers to consider key questions related to nucleation and crystallization, as well as chemically (spinodal decomposition) or density-driven (polyamorphism) phase separation, which could lead to future applications in a variety of materials.

Resolution enhancement in solid-state MQ-MAS experiments achieved by composite decoupling

It is shown that, in the presence of strong heteronuclear dipolar couplings, resolution can be significantly improved in solid‐state multiple quantum magic angle spinning NMR experiments on half integer quadrupolar nuclei by applying composite decoupling schemes during triple quantum evolution and acquisition. Reduction of the effects of heteronuclear dipolar coupling during the multiple quantum evolution period is shown to lead to improved resolution of 27Al sites in crystalline chiolite (Na5Al3F14) and 11B sites in a polyborazilene sample.

Continuous flow hyperpolarized 129Xe-MAS NMR studies of microporous materials

A magic angle spinning (MAS) NMR probe has been developed to allow in situ measurements of NMR spectra. Two applications are targeted with this device: i) in situ and operando MAS NMR spectroscopy of working catalysts and ii) the hyperpolarized (HP) 129Xe spectroscopy of porous materials under MAS and continuous flow conditions. The construction of the MAS probe is described and the usefulness of this system is demonstrated by studying the adsorption of hyperpolarized xenon on AlPO-41 and ITQ-6 zeolites. The high stability of the HP xenon flow allowed us to perform two-dimensional exchange experiments under MAS conditions, in a short time and with very good resolution.

In-situ interaction of cement paste and shotcrete with claystones in a deep disposal context

In-situ sampling was performed in the Andra Meuse/Haute Marne (France) Underground Research Laboratory (URL) allowing the study of two cement based materials/claystone interfaces that have undergone 4 to 5 years of interaction. The first interface concerned a shotcrete that was sprayed on the wall of an access drift at the laboratory level and the second one, a class G cement paste that was injected in a borehole filled from the surface and was intercepted during the excavation of a new gallery. In the first case, the hydrodynamic conditions were controlled by the ventilation of the drift; while in the second, the cement paste and claystone materials were considered saturated and far from any mechanical perturbation. A multi scale investigation was carried out to identify any evidence of alkaline perturbation in the cement based materials and the claystone. Chemical, mineralogical, and textural measurements were thus performed on the different materials in contact at the nanometer to a centimeter scale. Results showed that all the perturbations resulting from the geochemical contrast between the cement materials and the claystone were limited to a μm scale on each side of the interfaces. Carbonation was observed in the cement materials leading to an opening or a clogging of the porosity according to the hydrodynamic conditions and the formulation of the cement material. The distribution of the cation exchange population was also modified in the claystone in contact with the cement paste where a potassium saturation of the exchangeable fraction was identified. The originality of the present work is that realistic field controlled samples from the Andra URL were studied, hereby allowing the evaluation of the impact of natural heterogeneities of the in-situ experimental conditions (that is, hydrodynamic conditions, engineered damage zone, mineralogical variations) on the perturbations at the cement paste/claystone interfaces. An important result is that the clogging of porosity was not homogenous along the interfaces.

Unexpected similarities between the surface chemistry of cubic and hexagonal gallia polymorphs

γ-Ga2O3 and α-Ga2O3 samples were prepared as single-phase materials having a high surface area. 71Ga NMR showed the simultaneous presence of tetracoordinated (GaIV) and hexacoordinated (GaVI) in γ-Ga2O3, whereas in α-Ga2O3 most of the gallium present was found to be as GaVI. However, the apparent rate of methanol conversion into dimethyl ether (at 473 K) was nearly the same for both samples, when due account was taken of their respective surface area. The unexpected high activity of α-Ga2O3 was explained from IR spectroscopic results obtained by using acidity probes like CO and pyridine. These spectroscopic probe molecules showed, for both samples, the presence of strong Lewis acid sites related to coordinatively unsaturated (cus) GaIV ions which, for the case of α-Ga2O3, strongly suggests surface reconstruction. By contrast, CO2 adsorption revealed a higher basicity for the α-polymorph, as compared to γ-Ga2O3. This basicity can be directly correlated to a larger amount of surface GaVI ions which results in a higher degree of ionicity.

Synthesis and structure determination of CaSi1/3B2/3O8/3, a new calcium borosilicate

This article reports on the identification, synthesis, and in-situ structure determination of a new crystalline calcium borosilicate compound of composition CaSi(1/3)B(2/3)O(8/3). Synthesis was carried out by complete crystallization on annealing from a corresponding glassy composition in the widely studied CaO-SiO2-B2O3 ternary system. The crystallographic structure was determined ab initio using electron diffraction information and the charge flipping algorithm performed on synchrotron and neutron powder diffraction data collected in situ at high temperature. CaSi(1/3)B(2/3)O(8/3) is found to crystallize in the Pna2(1) (no. 33) orthorhombic space group, with a = 12.1025(4) Å, b = 5.2676(1) Å, c = 3.7132(1) Å, and V = 236.71(1) Å(3) at 650 °C. Solid-state (29)Si and (11)B NMR experiments have confirmed the existence of finite chains along the c axis, formed by corner-sharing SiO4 tetrahedra and BO3 units. Silicon and boron species share a crystallographic site, and the Si/B distribution induces different possible arrangements of the chains which are discussed in light of DFT calculations. At room temperature, the existence of a superstructure, resulting from the ordering within nanoscale domains, was explored by transmission electron microscopy.

1D to 3D NMR study of microporous alumino-phosphate AlPO4-40

From one‐ to two‐ and three‐dimensional MAS NMR solid‐state experiments involving 31P and 27Al, we show that the structure of microporous alumino‐phosphate AlPO4‐40 contains at least four times more sites than expected, and we attribute two types of AlIV sites. The newly described 27Al‐31P MQ‐HMQC opens new possibilities of describing details of three‐dimensional bounded networks. Copyright