Pierre Florian

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.

Multiple quantum magic-angle spinning using rotary resonance excitation

We have discovered rotary resonances between rf field strength, ω1, and magic-angle spinning (MAS) frequency, ωR, which dramatically enhance the sensitivity of triple quantum preparation and mixing in the multiple-quantum MAS experiment, particularly for quadrupolar nuclei having low gyromagnetic ratios or experiencing strong quadrupole couplings. Triple quantum excitation efficiency minima occur when 2ω1=nωR, where n is an integer, with significant maxima occurring between these minima. For triple quantum mixing we observe maxima when ω1=nωR. In both preparation and mixing the pulse lengths required to reach maxima exceed one rotor period. We have combined these rotary resonance conditions into a new experiment called FASTER MQ-MAS, and have experimentally demonstrated a factor of 3 enhancement in sensitivity in comparison to conventional MQ-MAS.

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 .

Nature and Structure of Aluminum Surface Sites Grafted on Silica from a Combination of High-Field Aluminum-27 Solid-State NMR Spectroscopy and First-Principles Calculations

The determination of the nature and structure of surface sites after chemical modification of large surface area oxides such as silica is a key point for many applications and challenging from a spectroscopic point of view. This has been, for instance, a long-standing problem for silica reacted with alkylaluminum compounds, a system typically studied as a model for a supported methylaluminoxane and aluminum cocatalyst. While (27)Al solid-state NMR spectroscopy would be a method of choice, it has been difficult to apply this technique because of large quadrupolar broadenings. Here, from a combined use of the highest stable field NMR instruments (17.6, 20.0, and 23.5 T) and ultrafast magic angle spinning (>60 kHz), high-quality spectra were obtained, allowing isotropic chemical shifts, quadrupolar couplings, and asymmetric parameters to be extracted. Combined with first-principles calculations, these NMR signatures were then assigned to actual structures of surface aluminum sites. For silica (here SBA-15) reacted with triethylaluminum, the surface sites are in fact mainly dinuclear Al species, grafted on the silica surface via either two terminal or two bridging siloxy ligands. Tetrahedral sites, resulting from the incorporation of Al inside the silica matrix, are also seen as minor species. No evidence for putative tri-coordinated Al atoms has been found.

A distribution of activation energies for the local and long-range ionic motion is consistent with the disordered structure of the perovskite Li3xLa2/3-xTiO3

Abstract 7 Li nuclear magnetic resonance spin–lattice relaxation time T1 versus temperature is reported in the 150 K–900 K temperature range on lithium lanthanum titanate fast ionic conductors. Because of the presence of disorder in the distribution of the lanthanum ions in the crystalline structure of this oxide and consequently in the conduction pathways of the lithium ions we propose to explain the strong asymmetry shown by these T1 versus 1/T curves by assuming independent ionic hops over a distribution of activation energies for the thermally activated Li+ ion hops. According to this assumed model the spin–lattice relaxation times T1 and the DC conductivity are fitted consistently in the 200–600 K and 300–400 K temperature ranges respectively. For both lower and higher temperatures a departure of the experimental data from the model is observed and explained. The use of this model to fit both T1 and DC conductivity data ruled out the possibility that different forms of the distribution would lead to a reasonable representation of T1. The physical meaning of the obtained parameters is discussed in accordance with the structure of the compounds.

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.

Structural and scintillation properties of new Ce3+-doped alumino-borate

The compound LuAl 3 (BO 3 ) 4 doped with Ce 3+ ions has been synthesized and investigated for the first time. It was compared with analogous Y and Gd alumino-borates. All materials have been prepared in the form of polycrystalline powders by solid state reaction. These three compounds have a trigonal structure with a R32 space group and the symmetry of each aluminium, lanthanide and boron site have been confirmed by MAS-NMR experiments. Main structural, thermal and fluorescence properties are given. The results show that in these compounds there are two sites of Ce 3+ and in the ultra violet range (maximum at 340 and 365 nm) Ce 3+ exhibits a 25 ns fluorescence. An extra-emission is observed in the low energy range. It could be originated from non-regular sites (aluminium and/or interstitial sites).

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.

Introduction of boron in hydroxyapatite: synthesis and structural characterization

Abstract Apatites doped with rare-earth ions have been extensively studied due to their potential applications as phosphors or laser hosts. The structure of apatite is based on a network of only tetrahedral PO 4 groups. When boron is added, phosphate and OH groups would be partially substituted by borate groups and new calcium borohydroxyapatite with nominal stoichiometry Ca 10 PO 4 6−x BO 3 x BO 3 y BO 2 z OH 2−3y−z is proposed. When P/B ratio=7.22, boron atoms are totally introduced in the apatitic lattice, but from P/B=11 samples are biphased borohydroxyapatite and Ca(OH) 2 and when P/B is lower than 7.22, Ca 3 (BO 3 ) 2 is also observed. The infra-red (IR) and Raman spectroscopy and 11 B MAS (magical angle spinning)–NMR experiments prove that boron is introduced as two-fold coordinated boron BO − 2 in the channels of the apatitic structure and as triangular BO 3− 3 groups substituting PO 4 and OH groups leading to a AB-type borohydroxyapatite. A comparison with a free boron hydroxyapatite shows that P and proton sites are split into several sites in the substituted compounds.

Nuclear magnetic resonance investigation of Li+-ion dynamics in the perovskite fast-ion conductor Li3xLa2/3-x□1/3-2xTiO3

7Li nuclear magnetic resonance relaxation times T1, T1ρ and T2 versus temperature are reported in the 150-900 K temperature range for the lithium lanthanum titanate Li3xLa2/3-x1/3-2xTiO3 perovskite-type fast-ionic conductors. The presence of Li+ ions of two kinds with slightly differing environments is displayed in these experiments. These ions exhibit two different motions: a fast one with a characteristic frequency around 100 MHz at 350 K and a slow one whose frequency is around 60 kHz at 280 K. These two different Li+ species cannot be differentiated by means of the fast motion (only one T1 is observed from the experiments), but only by means of the slow ones (two T1ρ and two T2 are observed). These motions are respectively attributed to Li+ motion inside the A-cage of the perovskite structure formed by the oxygen ions and to Li+ hops between the cages. T1- and T1ρ-studies also performed on the 6Li nucleus clearly show that just dipolar nuclear interaction is responsible for Li+ relaxation. This result is at variance with what has been previously put forward for the relaxation process in these compounds.