Dominique Massiot

Two-dimensional magic-angle spinning isotropic reconstruction sequences for quadrupolar nuclei.

Two-dimensional magic-angle spinning (triple quantum, single quantum) correlation pulse sequences and phase cycles based on the technique of Frydman and Harwood for the reconstruction of the isotropic spectrum of half-integer spin quadrupolar nuclei broadened to second-order are described. These sequences provide pure absorption mode two-dimensional lineshapes and increased sensitivity. Experimental examples on spin I = 3/2 (87Rb in RbNO3) and I = 5/2 (27Al in NaSi3AlO8) are presented. The isotropic chemical shift and quadrupolar coupling parameters could be obtained from a simple analysis of the triple quantum filtered single quantum magic-angle spinning cross-sections.

A quantitative study of 27Al MAS NMR in crystalline YAG

A quantitative 27Al NMR analysis of yttrium aluminum garnet. involving a complete computation of the quadrupolar interaction in MAS conditions, has been performed. Results show a very good agreement with the known X-ray structure: three tetrahedral aluminum sites and two octahedral. This analysis provides a new approach for obtaining quantitative results from MAS NMR spectra of quadrupolar nuclei in different environments.

71Ga and 69Ga nuclear magnetic resonance study of β-Ga2O3: resolution of four- and six-fold coordinated Ga sites in static conditions

71Ga and 69Ga nuclear magnetic resonance (NMR) spectra have been obtained for beta-Ga2O3 at magnetic fields of 11.7 and 7.0 T with a combination of static and spinning samples and using echo techniques. Isotropic chemical shifts and well constrained electric field gradient (EFG) tensors were measured for the GaIV and GaVI sites in beta-Ga2O3 and are reported for the first time. Due to its high quadrupolar coupling constant the GaIV site in beta-Ga2O3 is only resolved in static conditions. Analysis of spectral discontinuities obtained for different isotopes and magnetic field strengths and a method of readily obtaining very broad spectra without point by point acquisition demonstrate an approach that should be very useful in constraining the chemical shifts and quadrupole parameters involved in solid-state gallium NMR spectra.

MAS NMR spectra of quadrupolar nuclei in disordered solids: The Czjzek model

Structural disorder at the scale of two to three atomic positions around the probe nucleus results in variations of the EFG and thus in a distribution of the quadrupolar interaction. This distribution is at the origin of the lineshape tailing toward high fields which is often observed in the MAS NMR spectra of quadrupolar nuclei in disordered solids. The Czjzek model provides an analytical expression for the joint distribution of the NMR quadrupolar parameters upsilon(Q) and eta from which a lineshape can be predicted. This model is derived from the Central Limit Theorem and the statistical isotropy inherent to disorder. It is thus applicable to a wide range of materials as we have illustrated for 27Al spectra on selected examples of glasses (slag), spinels (alumina), and hydrates (cement aluminum hydrates). In particular, when relevant, the use of the Czjzek model allows a quantitative decomposition of the spectra and an accurate extraction of the second moment of the quadrupolar product. In this respect, it is important to realize that only rotational invariants such as the quadrupolar product can make sense to describe the quadrupolar interaction in disordered solids.

Exploring electrolyte organization in supercapacitor electrodes with solid-state NMR.

Supercapacitors are electrochemical energy-storage devices that exploit the electrostatic interaction between high-surface-area nanoporous electrodes and electrolyte ions. Insight into the molecular mechanisms at work inside supercapacitor carbon electrodes is obtained with (13)C and (11)B ex situ magic-angle spinning nuclear magnetic resonance (MAS-NMR). In activated carbons soaked with an electrolyte solution, two distinct adsorption sites are detected by NMR, both undergoing chemical exchange with the free electrolyte molecules. On charging, anions are substituted by cations in the negative carbon electrode and cations by anions in the positive electrode, and their proportions in each electrode are quantified by NMR. Moreover, acetonitrile molecules are expelled from the adsorption sites at the negative electrode alone. Two nanoporous carbon materials were tested, with different nanotexture orders (using Raman and (13)C MAS-NMR spectroscopies), and the more disordered carbon shows a better capacitance and a better tolerance to high voltages.

Chemical bonding differences evidenced from J-coupling in solid state NMR experiments involving quadrupolar nuclei

Small scalar J-coupling between quadrupolar nuclei and spin 1/2 can be measured in inorganic solids using J-Resolved experiments and further used to acquire 2D J-HQMC heteronuclear correlation, giving detailed insight into the chemical bonding scheme.

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.

Infrared reflectivity spectroscopy of silicate glasses

Infrared reflection spectra of more than fifty silicate glasses have been obtained with a Fourier-transform infrared spectrometer and are reported in the range from 20 to 4000 cm −1 . Results have been systematically analysed with both Kramers-Kronig inversion and fit of the factorized form of the dielectric function to reflectivity spectra. Good agreement between both methods has been found. The detailed analysis of this set of data shows localized modes characteristic of the response of cations like Na, K or Ca, as opposed to the coupled-mode behavior of Al-O-Si vibrations. The degree of coherence of vibrations is also discussed. The specific potentialities of this spectroscopy and the kind of conclusions that can be drawn about the structure of silica glasses — when complete spectra are available — is illustrated by several examples in silica glasses containing Na, K, Ca, Al, Zr, F, B or Mg ions.

A structural investigation of CaOAl2O3 glasses via 27Al MAS-NMR

Abstract 27Al isotropic chemical shifts and quadrupolar coupling constants have been obtained for glasses along the CaOAl2O3 join, prepared by fast (splat) quench techniques and slow cooling in containerless levitation experiments. Quench rates with the two techniques were 106-107°/s and 200–300°/s. Fast-quenched glasses with CaO:Al2O3

Crystal structure of κ-alumina: an X-ray powder diffraction,TEM and NMR study

The crystal structure of κ-alumina (κ-Al 2 O 3 ) has been determined ab initio from an X-ray powder diffraction pattern (reliability factors: R Bragg =0.046, R p =0.090, R wp =0.115, χ 2 =11.7). The acentric structure (orthorhombic system, space group Pna2 1 , a=4.8437(2) A, b=8.3300(3) A, c=8.9547(4) A, Z=8) is built up from a pseudo-close-packed stacking ABAC of oxygen atoms, with aluminium in octahedral and tetrahedral environments in a 3:1 ratio, which form zigzag ribbons of edge-sharing octahedra and corner-sharing tetrahedra. κ-Alumina was characterized using magic angle spinning (MAS) 27 Al NMR at two fields and multiple quantum magic angle spinning (3Q MQ MAS) 27 Al NMR. A high-resolution electron microscopy study confirmed the structure and showed the presence of two types of defects: antiphase boundaries and 120 ° disorientations. A model is proposed for these two types of defects, which leaves unchanged the pseudo-close-packed arrangement of the oxygen atoms and assumes a shift or 120 ° twinning of aluminium ions.