Scott Kroeker

Quantification of five- and six-coordinated aluminum ions in aluminosilicate and fluoride-containing glasses by high-field, high-resolution 27Al NMR

Aluminum cation sites with five ([5]Al) or six ([6]Al) anion neighbors are minor species in aluminosilicate glasses that are interesting in models of structure, diffusion, and viscous flow. Using 27Al NMR, we present the first direct evidence for [5]Al in a calcium-aluminosilicate glass without excess aluminum over charge-balancing cations, and quantify small concentrations of both [5]Al and [6]Al in several fluoride-containing aluminosilicate glasses. NMR techniques that enhance resolution by decreasing the effects of second-order quadrupolar broadening, including triple-quantum magic-angle spinning (3QMAS), analysis of spinning sidebands, and data collection at very high external magnetic fields (14.1 and 18.8 T), are particularly effective in accurately observing such species.

Oxygen triclusters in crystalline CaAl4O7 (grossite) and in calcium aluminosilicate glasses: 17O NMR

Abstract We present 17O MAS NMR data for crystalline calcium dialuminate (grossite), CaAl2O4 and monoaluminate, CaAl4O7. The first of these contains an oxygen tricluster site and serves as a model compound for sites of this type in aluminosilicate glasses. Tricluster site NMR parameters are distinct from those of bridging O atoms (Al-O-Al), allowing partial resolution in triple quantum MAS NMR spectra. Such spectra for calcium aluminosilicate glasses are consistent with the presence of a small fraction of tricluster sites. Observed chemical shifts for non-bridging oxygen (NBO) atoms in an impurity phase in the CaAl2O4 sample are distinct from those for NBO in Ca-aluminosilicate glasses, indicating that the latter are primarily bonded to Si, not Al.

Non-bridging oxygen sites in barium borosilicate glasses: results from 11B and 17O NMR

Abstract The concentration and structural role of non-bridging oxygen (NBO), defined as oxygen bonded to one framework cation, play a major role in the physical properties of glasses and liquids. In this study, the distribution of NBO connected to either silicon and boron was investigated in a borosilicate glass system using 17 O magic-angle spinning (MAS) and triple quantum (3QMAS) nuclear magnetic resonance. In order to obtain better separation of different 17 O NBO resonances, barium borosilicate glasses were chosen and measured by MAS and 3QMAS. The two-dimensional 3QMAS spectra show clear resolution between peaks for Si–NBO and B–NBO, allowing their relative proportions to be at least roughly quantified for the first time. The NMR parameters for these sites were also determined and are consistent with previous studies of binary barium borate and silicate systems. Site populations are significantly different from predictions of conventional models based on alkali borosilicates, which suggests a more disordered nature for these barium borosilicate systems. In order to explain the anomalously high NBO concentrations, we hypothesize that a few percent of the oxygens may have three network cation neighbors, forming ‘triclusters’.

Magnesium coordination environments in glasses and minerals: New insight from high-field magnesium-25 MAS NMR

Abstract A comparison of 25Mg magic angle spinning (MAS) NMR spectra of crystalline and glassy diopside (CaMgSi2O6) reveals that the chemical shift in the disordered phase corresponds to that of the mineral, suggesting that sixfold coordination is essentially retained upon vitrification. Likewise, a crystalline leucite analogue (K2MgSi5O12) known to possess fourfold-coordinated Mg has the same peak position as the corresponding glass. In addition to being a sensitive probe of local structure by analogy with crystalline phases, these data may be understood in terms of the effect of competing cation field strengths: competing network modifier cations with higher field strength induce higher magnesium coordination numbers, with a consequent increase in bond length and decrease in chemical shift. This work demonstrates the utility of NMR at 14.1 Tesla for probing the coordination environment of Mg in glasses and minerals, and suggests great potential for high-field NMR investigations of quadrupolar nuclides with low resonance frequencies in amorphous systems.

Non-bridging oxygens in borate glasses: characterization by 11B and 17O MAS and 3QMAS NMR.

The concentrations of non-bridging oxygens (NBO) in oxide glasses has major effects on their properties and on those of their precursor glass melts. In borate and borosilicate glasses, the presence of NBO bonded to boron has generally been inferred from 11B NMR spectra and mass balance considerations. Here we report the direct observation of such NBO using 17O MAS and 3QMAS techniques, and compare estimates of their populations with those derived from high-resolution 11B MAS spectra. For the latter, two independent methods are used, based on the ratios of trigonal to tetrahedral boron and on the concentrations of trigonal boron sites with large quadrupolar asymmetry parameters. We include data on crystalline sodium pyroborate (Na4B2O5) and sodium metaborate (NaBO2), and several sodium and barium borate glasses. 17O chemical shifts and quadrupolar coupling constants for NBO bonded to boron vary considerably depending on their coordination environment. In borosilicates, peaks for this species may be hidden by overlap with B-O-Si or Si-O-Si resonances.

The mechanism of solution of aluminum oxide in MgSiO3 perovskite

We report 27Al and 29Si Nuclear Magnetic Resonance (NMR) spectra, collected at magnetic fields of 14.1 and 18.8 Tesla on samples as small as 1 mg, for Al-bearing MgSiO3 perovskite synthesized at 26–28 GPa. For Al, we find a 1∶1 ratio of two types of sites: a symmetrical, octahedral site and a low symmetry, distorted site that is most likely to be Al in modifications of normally eight-coordinated Mg sites. A charge coupled substitution of 2 Al for one Si and one Mg cation is strongly supported as the predominant mechanism in this pressure range.

Mushroom elbaite from the Kat Chay mine, Momeik, near Mogok, Myanmar: I. Crystal chemistry by SREF, EMPA, MAS NMR and Mössbauer spectroscopy

Abstract Tourmaline from the Kat Chay mine, Momeik, near Mogok, Shan state, Myanmar, shows a variety of habits that resemble mushrooms, and it is commonly referred to as ‘mushroom tourmaline’. The structure of nine single crystals of elbaite, ranging in colour from pink to white to black and purple, extracted from two samples of mushroom tourmaline from Mogok, have been refined (SREF) to R indices of ~2.5% using graphite-monochromated Mo-Kα X-radiation. 11B and 27Al Magic Angle Spinning Nuclear Magnetic Resonance spectroscopy shows the presence of [4]B and the absence of [4]Al in samples with transition-metal content low enough to prevent paramagnetic quenching of the signal. Site populations were assigned from refined site-scattering values and unit formulae derived from electron-microprobe analyses of the crystals used for X-ray data collection. 57Fe Mössbauer spectroscopy shows that both Fe2+ and Fe3+ are present, and the site populations derived by structure refinement show that there is no Fe at the Z site; hence all Fe2+ and Fe3+ occurs at the Y site. The 57Fe Mössbauer spectra also show peaks due to intervalence charge-transfer involving Fe2+ and Fe3+ at adjacent Y sites. Calculation of the probability of the total amount of Fe occurring as Fe2+–Fe3+ pairs for a random short-range distribution is in close accord with the observed amount of Fe involved in Fe2+-Fe3+, indicating that there is no short-range order involving Fe2+ and Fe3+ in these tourmalines.

The occurrence of tetrahedrally coordinated Al and B in tourmaline: An 11B and 27Al MAS NMR study

Abstract Considerable uncertainty has surrounded the occurrence of tetrahedrally coordinated Al and B at the T site in tourmaline. Although previously detected in several tourmaline specimens, the frequency of these substitutions in nature, as well as the extent to which they occur in the tourmaline structure, is not known. Using 11B and 27Al MAS NMR spectroscopy, we have investigated the presence of B and Al at the T site in 50 inclusion-free tourmaline specimens of low transition-metal content and different species (elbaite, “fluor-elbaite” , liddicoatite, dravite, uvite, olenite, and magnesiofoitite) from different localities worldwide. Chemical shifts of [4]B and [3]B in 11B spectra, and [4]Al and [6]Al in 27Al spectra, are well resolved, allowing detection of even small amounts of T-site constituents. In the observed spectra, [4]B and [3]B peaks are located at 0 and 18-20 ppm, respectively, with the greatest intensity corresponding to [3]B (=3 apfu). In 27Al spectra, [4]Al and [6]Al bands are located at 68-72 and 0 ppm, respectively, with the greater intensity corresponding to [6]Al. However, inadequate separation of YAl and ZAl precludes resolution of these two bands. Simulation of 11B MAS NMR spectra shows that tetrahedrally and trigonally coordinated B can be readily distinguished at 14.1 T and that a [4]B content of 0.0-0.5 apfu is common in tourmaline containing low amounts of paramagnetic species. 27Al MAS NMR spectra show that Al is also a common constituent of the T site in tourmaline. Determination of [4]Al content by peak-area integration commonly shows values of 0.0-0.5 apfu. Furthermore, the chemical shift of the 27Al tetrahedral peak is sensitive to local order at the adjacent Y and Z octahedra, where [4]Al-YMg3 and [4]Al-Y(Al,Li)3 arrangements result in peaks located at ~65 and ~75 ppm, respectively. Both 11B MAS NMR and 27Al MAS NMR spectra show peak broadening as a function of transition-metal content (i.e., Mn2+ + Fe2+ = 0.01-0.30 apfu) in the host tourmaline. In 11B spectra, broadening and loss of intensity of the [3]B signal ultimately obscures the signal corresponding to [4]B, increasing the limit of detection of [4]B in tourmaline. Our results clearly show that all combinations of Si, Al, and B: T = (Al, Si)6, T = (B, Si)6, T = (Al, B, Si)6, and T = Si6 apfu, are common in natural tourmalines.

Enhanced resolution and quantitation from `ultrahigh' field NMR spectroscopy of glasses

Abstract The utilization of higher magnetic fields for nuclear magnetic resonance (NMR) (e.g., 14.1 and 18.8 T) can enhance both resolution and sensitivity in studies of disordered materials. When used in conjunction with fast magic angle spinning (MAS), high-field NMR can yield structural information not always available from NMR experiments of glasses at lower fields. Examples include site distinction of multiple boron groupings in 11 B MAS NMR spectra of borate glasses, the spectroscopic observation of low concentrations of five- and six-co-ordinate aluminum species in 27 Al MAS NMR spectra of aluminosilicate glasses, and new insight into magnesium co-ordination environments in silicate glasses using 25 Mg MAS NMR. Also presented are 17 O MAS NMR spectra of a zeolite in which high-field experiments permit resolution and quantification of bound molecular water. Comparisons of spectral data collected at 18.8 T with those at lower fields demonstrate these principles and indicate the potential of high-field MAS NMR applications to amorphous solids.

Magnetic properties of the geometrically frustrated S= 1 2 antiferromagnets, La2 LiMoO6 and Ba2 YMoO 6, with the B-site ordered double perovskite structure: Evidence for a collective spin-singlet ground state

Two B-site ordered double perovskites, La2LiMoO6 and Ba2YMoO6, based on the S = 1/2 ion, Mo5+, have been investigated in the context of geometric magnetic frustration. Powder neutron diffraction, heat capacity, susceptibility, muon spin relaxation(_SR), and 89Y NMR- including MAS NMR- data have been collected. La2LiMoO6 deviates strongly from simple Curie-Weiss paramagnetic behavior below 150K and zero-field cooled/ field cooled (ZFC/FC)irreversibility occurs below 20K with a weak, broad susceptibility maximum near 5K in the ZFC data. A Curie-Weiss fit shows a reduced mu_eff=1.42\mu_B, (spin only = 1.73 muB) and a Weiss temperature, \theta_c, which depends strongly on the temperature range of the fit. Powder neutron diffraction, heat capacity and 7Li NMR show no evidence for long range magnetic order to 2K. On the other hand oscillations develop below 20K in muSR indicating at least short range magnetic correlations. Susceptibility data for Ba2YMoO6 also deviate strongly from the C-W law below 150K with a similarly reduced mu_eff = 1.72\mu_B and \theta_c = - 219(1)K. Heat capacity, neutron powder diffraction and muSR data show no evidence for long range order to 2K but a very broad maximum appears in the heat capacity. The 89Y NMR paramagnetic Knight shift shows a remarkable local spin susceptibility behavior below about 70K with two components from roughly equal sample volumes, one indicating a singlet state and the other a strongly fluctuating paramagnetic state. Further evidence for a singlet state comes from the behavior of the relaxation rate, 1/T1. These results are discussed and compared with those from other isostructural S = 1/2 materials and those based on S = 3/2 and S = 1.