Atomic scale structure of amorphous aluminum oxyhydroxide, oxide and oxycarbide films probed by very high field 27Al nuclear magnetic resonance.

The atomic scale structure of aluminum in amorphous alumina films processed by direct liquid injection chemical vapor deposition from aluminum tri-isopropoxide (ATI) and dimethyl isopropoxide (DMAI) is investigated by solid-state 27Al nuclear magnetic resonance (SSNMR) using a very high magnetic field of 20.0 T. This study is performed as a function of the deposition temperature in the range 300-560 °C, 150-450 °C, and 500-700 °C, for the films processed from ATI, DMAI (+H2O), and DMAI (+O2), respectively. While the majority of the films are composed of stoichiometric aluminum oxide, other samples are partially or fully hydroxylated at low temperature, or contain carbidic carbon when processed from DMAI above 500 °C. The quantitative analysis of the SSNMR experiments reveals that the local structure of these films is built from AlO4, AlO5, AlO6 and Al(O,C)4 units with minor proportions of the 6-fold aluminum coordination and significant amounts of oxycarbides in the films processed from DMAI (+O2). The aluminum coordination distribution as well as the chemical shift distribution indicate that the films processed from DMAI present a higher degree of structural disorder compared to the films processed from ATI. Hydroxylation leads to an increase of the 6-fold coordination resulting from the trend of OH groups to integrate into AlO6 units. The evidence of an additional environment in films processed from DMAI (+O2) by 27Al SSNMR and first-principle NMR calculations on Al4C3 and Al4O4C crystal structures supports that carbon is located in Al(O,C)4 units. The concentration of this coordination environment strongly increases with increasing process temperature from 600 to 700 °C favoring a highly disordered structure and preventing from crystallizing into γ-alumina. The obtained results are a valuable guide to the selection of process conditions for the CVD of amorphous alumina films with regard to targeted applications.

77Se solid-state NMR investigations on As(x)Se(1-x) glasses using CPMG acquisition under MAS.

(77)Se (I=1/2) solid-state NMR is a very sensitive probe of the local structure of selenide glasses, which themselves are promising for optical applications. In this work, we show that although (77)Se has a low natural abundance (7.58%) and a wide spectral range, the sensitivity can be dramatically increased using Carr-Purcell-Meiboom-Gill (CPMG) trains of rotor-synchronized π pulses during the detection of (77)Se magnetization but may be affected by chemical shift anisotropy when the Magic Angle Spinning rate is not fast enough and by offset effects. The indirect dimension of the T(2)(CPMG)-resolved spectrum shows a strong influence of the J-couplings between naturally occurring (77)Se pairs. The resulting spectra show that the structural model known as "chains crossing model" is not entirely suitable to describe the glassy network of the Se-rich compositions.

91Zr nuclear magnetic resonance spectroscopy of solid zirconium halides at high magnetic field.

(91)Zr solid-state NMR spectra of zirconium halides and several fluorozirconates have been obtained at high magnetic fields up to 30 T using both the Hahn-Echo and the Quadrupolar Carr-Purcell-Meiboom-Gill sequences combined with the broadband Variable Offset Cumulative Spectrum technique. For the zirconium halides, the (91)Zr isotropic chemical shift covers a range of about 2000 ppm and shows a good correlation with Pauling's electronegativity and ionic potential of the halogen. For the fluorozirconate samples, in which the Zr atoms exhibit various coordination polyhedra, increasing the Zr coordination number and the mean Zr-F bond length leads to an increased isotropic shielding. In the studied compounds the (91)Zr quadrupolar coupling constants (C(Q)'s) range from 10.6 to 44.7 MHz. For 6-fold coordinated Zr sites, a correlation between C(Q) and the shear strain of the octahedron is observed, and we investigate the relationship between the C(Q) and the distortion of the polyhedron for 8-fold coordinated Zr sites using different distortion criteria.

27Al NMR study of the structure of lanthanum- and yttrium-based aluminosilicate glasses and melts.

We have investigated by (27)Al nuclear magnetic resonance spectroscopy some compositions in the Ln(2)O(3)-Al(2)O(3)-SiO2 (Ln = Y or La) ternary phase diagram containing more than 60 mol % of SiO2. One- and two-dimensional high-field (17.6 T) high-speed (30 kHz) magic angle spinning experiments have been performed along with simulations of the spectra to quantify the amount of penta-coordinated aluminum present in those glasses as a function of composition. Very high-temperature experiments have allowed to follow selected samples from 2200 degrees C down to 1700 degrees C and hence to characterize the aluminum coordination state and dynamics in those liquids. The present study re-enforces the current view that "minor" species such as penta-coordinated aluminum are actually present in a considerable amount in aluminosilicate glasses, and high-temperature liquids at and above the charge compensation join. The high-field strength of Y3+ and La3+ reveal, for the first time in glasses, a different mean electric field gradient perceived by the tetra- and penta-coordinated aluminum environments. The movements responsible for the NMR relaxation of aluminum in the high-temperature liquid are shown to be uncorrelated with the movements responsible for the macroscopic shear viscosity. Results obtained both on glasses and in situ at high-temperature suggest a preferential localization of Ln3+ nearby tetra-coordinated aluminum species, with possible formation of tricluster and/or Ln3+ coordination changes.

19F high magnetic field NMR study of beta-ZrF4 and CeF4: from spectra reconstruction to correlation between fluorine sites and 19F isotropic chemical shifts.

High magnetic field and high spinning frequency one- and two-dimensional one-pulse MAS 19F NMR spectra of beta-ZrF4 and CeF4 were recorded and reconstructed allowing the accurate determination of the 19F chemical shift tensor parameters for the seven different crystallographic fluorine sites of each compound. The attributions of the NMR resonances are performed using the superposition model for 19F isotropic chemical shift calculation initially proposed by Bureau et al. (Bureau, B.; Silly, G.; Emery, J.; Buzaré, J.-Y. Chem. Phys. 1999, 249, 85-104). A satisfactory reliability is reached with a root-mean-square (rms) deviation between calculated and measured isotropic chemical shift values equal to 1.5 and 3.5 ppm for beta-ZrF4 and CeF4, respectively.

Application of the MAS-J-HMQC experiment to a new pair of nuclei {29Si,31P}: Si5O(PO4)6 and SiP2O7 polymorphs.

We report the results of the two-dimensional MAS-J-HMQC experiment providing scalar correlations between 29Si and 31P nuclei in solid state NMR, and we give the first evaluation of the 2JSi-O-P coupling constants (approximately 15 Hz) for a crystalline silicophosphate phase Si5O(PO4)6. The experiment is applied to the characterization of complex mixtures of SiP2O7 phases, through editing of 31P spin pairs by the heteronuclear 2JP-O-Si interaction.

Measurement of J-couplings between spin-(1/2) and quadrupolar nuclei by frequency selective solid-state NMR.

We report a REDOR-based scheme for the measurement of heteronuclear J-couplings in solid samples with well defined structure, containing spin-12 and quadrupolar nuclei, which can be used with selective RF irradiation to target a specific spin pair, and which provides direct information about the number of coupled spins.

77Se and 87Rb Solid state NMR study of the structure of Rb2[Pd(Se4)2].Se8.

The structure of Rb(2)[Pd(Se(4))(2)].Se(8) has been investigated using (87)Rb magic angle spinning and static NMR and (77)Se magic angle spinning NMR. The number and the integrated intensities of the (87)Rb and (77)Se resonances are in full agreement with the crystallographic structure of the compound. The (87)Rb and (77)Se nuclear spin interaction parameters have been used to characterize the main structural units of the compound: infinite [Rb(Se(8))](x)(x+) columns and polymeric [Pd(Se(4))(2)](x)(2x-) sheet anions.

Solid-state 1H and 27Al NMR studies of amorphous aluminum hydroxides.

Two kinds of amorphous aluminum hydroxides, a sample precipitated from admixing AlCl3 and NaOH aqueous solutions and the commercial product, were measured by 27Al and 1H solid-state NMR spectroscopy. Pentahedral and tetrahedral coordinations, as well as octahedral coordination of oxygen atoms for aluminum, are observed in 27Al magic angle spinning (MAS) spectra of both amorphous samples. In contrast, octahedral coordination is only observed in gibbsite, bayerite, and boehmite. According to 1H MAS-NMR spectra under conditions of high spinning rate (35 kHz) and high field (14.09 T), free waters and OH groups coupled with aluminum for amorphous samples are observed at approximately 5 and approximately 4.5 ppm, respectively, the latter peak being broader. This is consistent with the differential spectra between spin echo and transfer of populations in double resonance. We conclude that the subunits of AlO4, AlO5, and AlO6 in amorphous aluminum hydroxides are bound through hydrogen bonds with a wide distribution of bonding strength.

New pillared layered gallium phosphonates in the gallium/1,2-ethylenediphosphonic acid system.

1,2-Ethylenediphosphonic acid reacts with gallium nitrate in water to give new pillared layered gallium phosphonates, the structure of which depends on the experimental conditions used for their preparation. Thus, Ga(4)(O(3)PC(2)H(4)PO(3))(3) (1) and Ga(2)(OH)(2)(O(3)PC(2)H(4)PO(3)) (2) were isolated and the structure of 1 was solved by single-crystal X-ray diffraction, while 2 was shown to have the same metal/PO(3) arrangement as the previously described Ga(OH)(O(3)PCH(3)). Ga(4)(O(3)PC(2)H(4)PO(3))(3) (1) is triclinic, with space group P1 with Z = 2, a = 5.1480(4), b = 8.0354(7), and c = 12.383(1) A; alpha = 91.34(1), beta = 101.40(1), and gamma = 90.86(1) degrees; V = 501.9(1) A(3). The structure of 1 is unusual with (i) mixed GaO(4) and GaO(5) sites while a 6-fold coordination is observed for gallium in (2) and (ii) the presence of the organic moiety within both the layers and the interlayer space. The two compounds were fully characterized using (31)P and (71)Ga solid-state NMR.

A space group assignment of ZrP2O7 obtained by 31P solid state NMR.

2-D 31P dipolar recoupling magic angle spinning NMR has been used to determine the true symmetry of the low temperature structure of ZrP2O7 for the first time.

Characterization of an Al-, Ga-based catalyst by Ga NMR and XAS.

Both Nuclear Magnetic Resonance (NMR) and X-Ray absorption spectroscopies characterize local order around the observed nuclei. With the recent progresses of 71Ga solid state NMR, it has become possible to take advantage of the complementary information that can be obtained using these two methods. This opens the possibility of a more thorough description of the first coordination shells of Ga in oxide. We present and discuss the example of an Al-, Ga-based catalyst.

Pure absorption-mode spectra using a modulated RF mixing period in MQMAS experiments.

Different approaches to obtain pure absorption-mode lineshapes in MQMAS experiments employing a train of 180 degrees phase-alternating pulses for the multiple-quantum to single-quantum mixing period are investigated. Four pulse sequences, which achieve this by using either the shifted-echo approach or the hypercomplex approach with symmetric coherence transfer pathways, are presented and their improved lineshape- and sensitivity-performance is experimentally demonstrated by (87)Rb MQMAS of RbNO(3). Compared to the original modulated-rf mixing sequence, sensitivity enhancements by factors up to 1.3 are obtained with the sequences described here.

From crystalline to glassy gallium fluoride materials: an NMR study of 69Ga and 71Ga quadrupolar nuclei.

Owing to the implementation of acquisition techniques specific for nuclei with very large quadrupolar interaction (full shifted echo and variable offset cumulative spectra (VOCS)), NMR spectra of 69Ga and 71Ga are obtained in crystallised (PbGaF5, Pb3Ga2F12, Pb9Ga2F24 and CsZnGaF6) and glassy (PbF2-ZnF2-GaF3) gallium fluorides. Simulations of both static (full echo or VOCS) and 15 kHz MAS spectra allow to obtain consistent determinations of isotropic chemical shifts and very large quadrupolar parameters (nu(Q) up to 14 MHz). In the crystalline compounds whose structures are unknown, the number and the local symmetry of the different gallium sites are tentatively worked out. For the glassy systems, a continuous Czjzek's distribution of the NMR quadrupolar parameters accounts for the particular shape of the NMR spectrum.

Chemical bonding of lead in glasses through isotropic vs anisotropic correlation: PASS shifted echo.

When observing spin I = (1/2) nuclei with important chemical shift anisotropy in disordered materials, the distribution of isotropic shift can become so large that no accessible spinning rate is able to provide a resolved spectrum. This is the case of 207Pb in glasses where static and high-speed MAS spectra are nearly identical. It is still possible in such a case to rebuild a spinning sideband free spectrum using a shifted echo modified PASS sequence. This makes it possible to discuss isotropic and anisotropic chemical shifts of lead in phosphate glasses, to characterize its structural role and its chemical bonding state.

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.

From crystalline to glassy gallium fluoride materials: an NMR study of 69Ga and 71Ga quadrupolar nuclei.

Owing to the implementation of acquisition techniques specific for nuclei with very large quadrupolar interaction (full shifted echo and variable offset cumulative spectra (VOCS)), NMR spectra of 69Ga and 71Ga are obtained in crystallised (PbGaF5, Pb3Ga2F12, Pb9Ga2F24 and CsZnGaF6) and glassy (PbF2-ZnF2-GaF3) gallium fluorides. Simulations of both static (full echo or VOCS) and 15 kHz MAS spectra allow to obtain consistent determinations of isotropic chemical shifts and very large quadrupolar parameters (nuQ up to 14 MHz). In the crystalline compounds whose structures are unknown, the number and the local symmetry of the different gallium sites are tentatively worked out. For the glassy systems, a continuous Czjzek's distribution of the NMR quadrupolar parameters accounts for the particular shape of the NMR spectrum.

14N NMR in AlN and BN.

A characterisation by 14N NMR of the binary nitrides AlN and BN is presented. Both the static and magic angle spinning (MAS) lineshapes have been investigated in order to determine, or set upper limits on, the nuclear quadrupole coupling (Cq) at the nitrogen site. Additional data are given for the Cq values at the Al and B sites. A comparison is made with other similar (mainly wurtzite) binary compounds for which Cq is known at each atomic site.

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 time resolved 27Al NMR study of the cooling process of liquid alumina from 2450 degrees C to crystallisation.

Nuclear magnetic resonance is able to give insight into the structure and dynamics of liquids at very high temperature (T > 2000 degrees C). 27Al NMR spectra have been recorded every 25 ms during the cooling of an aerodynamically levitated liquid alumina droplet from 2450 degrees C to crystallisation in less than 3 s. The temperature is measured jointly by pyrometry and NMR, and this time resolved experiment provides a unique way of exploring the temperature dependence of both the structure (shift) and the dynamics (relaxation time of 27Al of the liquid and the supercooled liquid alumina until the crystallisation of alpha-Al2O3. The apparent Arrhenian activation energy of 130 kJ mol-1, derived from T1 measurement, is understood as the signature of the macroscopic viscosity in this high temperature liquid.