Characterization of the Binary Nitrides VN and ScN by Solid-State NMR Spectroscopy

NMR spectra of polycrystalline samples of the binary nitrides ScN and VN were acquired under magic-angle spinning. The observed nuclides Sc-45, V-51 and N-14 are all quadrupolar nuclei with a spin I>1/2 ${I\char62 1/2}$ . However, due to the high symmetry of their rock-salt type structures, the spectra of the nitrides do not exhibit effects of quadrupolar or other anisotropic interactions of significant magnitude. This allows a relatively straightforward evaluation of the acquired spectra, leading to isotropic chemical shift values (delta(iso)) of -213 ppm ((VN)-V-51), 378 ppm ((VN)-N-14), 290 ppm ((ScN)-Sc-45) and 442 ppm ((ScN)-N-14) against commonly used reference standards. In the wider context of N-14-NMR of binary nitrides, it is shown that the distance of nitrogen to the nearest neighbour cation can be correlated to the observed chemical shift

Characterisation of contact twinning for cerussite, PbCO3, by single-crystal NMR spectroscopy

<jats:title>Abstract</jats:title><jats:p>Cerussite, <jats:inline-formula><jats:alternatives><jats:tex-math>$$\\hbox {PbCO}_3$$</jats:tex-math><mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\"> <mml:msub> <mml:mtext>PbCO</mml:mtext> <mml:mn>3</mml:mn> </mml:msub> </mml:math></jats:alternatives></jats:inline-formula>, like all members of the aragonite group, shows a tendency to form twins, due to high pseudo-symmetry within the crystal structure. We here demonstrate that the twin law of a cerussite contact twin may be established using only <jats:inline-formula><jats:alternatives><jats:tex-math>$$^{207}$$</jats:tex-math><mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\"> <mml:msup> <mml:mrow /> <mml:mn>207</mml:mn> </mml:msup> </mml:math></jats:alternatives></jats:inline-formula>Pb-NMR spectroscopy. This is achieved by a global fit of several sets of orientation-dependent spectra acquired from the twin specimen, allowing to determine the relative orientation of the twin domains. Also, the full <jats:inline-formula><jats:alternatives><jats:tex-math>$$^{207}$$</jats:tex-math><mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\"> <mml:msup> <mml:mrow /> <mml:mn>207</mml:mn> </mml:msup> </mml:math></jats:alternatives></jats:inline-formula>Pb chemical shift tensor in cerussite at room temperature is determined from these data, with the eigenvalues being <jats:inline-formula><jats:alternatives><jats:tex-math>$$\\delta _{11} = (-2315\\pm 1)$$</jats:tex-math><mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\"> <mml:mrow> <mml:msub> <mml:mi>δ</mml:mi> <mml:mn>11</mml:mn> </mml:msub> <mml:mo>=</mml:mo> <mml:mrow> <mml:mo>(</mml:mo> <mml:mo>-</mml:mo> <mml:mn>2315</mml:mn> <mml:mo>±</mml:mo> <mml:mn>1</mml:mn> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math></jats:alternatives></jats:inline-formula> ppm, <jats:inline-formula><jats:alternatives><jats:tex-math>$$\\delta _{22} = (-2492 \\pm 3)$$</jats:tex-math><mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\"> <mml:mrow> <mml:msub> <mml:mi>δ</mml:mi> <mml:mn>22</mml:mn> </mml:msub> <mml:mo>=</mml:mo> <mml:mrow> <mml:mo>(</mml:mo> <mml:mo>-</mml:mo> <mml:mn>2492</mml:mn> <mml:mo>±</mml:mo> <mml:mn>3</mml:mn> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math></jats:alternatives></jats:inline-formula> ppm, and <jats:inline-formula><jats:alternatives><jats:tex-math>$$\\delta _{33} = (-3071 \\pm 3)$$</jats:tex-math><mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\"> <mml:mrow> <mml:msub> <mml:mi>δ</mml:mi> <mml:mn>33</mml:mn> </mml:msub> <mml:mo>=</mml:mo> <mml:mrow> <mml:mo>(</mml:mo> <mml:mo>-</mml:mo> <mml:mn>3071</mml:mn> <mml:mo>±</mml:mo> <mml:mn>3</mml:mn> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math></jats:alternatives></jats:inline-formula> ppm.</jats:p&gt

Local Electronic Structure in AlN Studied by Single-Crystal ²⁷Al and ¹⁴N NMR and DFT Calculations

Both the chemical shift and quadrupole coupling tensors for 14 N and 27 Al in the wurtzite structure of aluminum nitride have been determined to high precision by single-crystal NMR spectroscopy. A homoepitaxially grown AlN single crystal with known morphology was used, which allowed for optical alignment of the crystal on the goniometer axis. From the analysis of the rotation patterns of 14 N ( I=1 ) and 27 Al ( I=5/2 ), the quadrupolar coupling constants were determined to χ(14N)=(8.19±0.02) kHz, and χ(27Al)=(1.914±0.001) MHz. The chemical shift parameters obtained from the data fit were δiso=−(292.6±0.6) ppm and δΔ=−(1.9±1.1) ppm for 14 N, and (after correcting for the second-order quadrupolar shift) δiso=(113.6±0.3) ppm and δΔ=(12.7±0.6) ppm for 27 Al. DFT calculations of the NMR parameters for non-optimized crystal geometries of AlN generally did not match the experimental values, whereas optimized geometries came close for 27 Al with χ¯¯calc=(1.791±0.003) MHz, but not for 14 N with χ¯¯calc=−(19.5±3.3) kHz

Green‐Emitting Oxonitridoberyllosilicate Ba[BeSiON2]:Eu2+ for Wide Gamut Displays

Light-emitting diodes (LEDs) producing pure, highly saturated colors are the industry standard for efficient backlighting of high-color gamut displays. Vivid color reproduction, matching the eye's perception of nature, is the central paradigm in the design of narrow-band emitting phosphors. To cover a wide range of naturally occurring color tones, expansion of the color gamut in the green spectral region, and therefore an advanced applicable green phosphor, is highly desired. Herein, the oxonitridoberyllosilicate Ba[BeSiON2]:Eu2+ showing outstanding narrow-band green emission (λmax ≈526 nm with FWHM ≈1600 cm−1 (≈45 nm), x = 0.212, y = 0.715) when excited with InGaN-based blue LEDs is presented. High quantum efficiency and low thermal quenching (>90% rel. quantum efficiency at 100 °C) as well as excellent scalability make the material suitable for industrial application in high color-gamut LED displays. A prototype phosphor-converted-LED (pc-LED), with green-emitting Ba[BeSiON2]:Eu2+ and K2SiF6:Mn4+ as red phosphor shows an extraordinary coverage in the CIE 1931 color space of 109% compared to the DCI-P3 standard, topping the widely applied β-SiAlON:Eu2+ phosphor (104%), making it suitable for use in phone displays, monitors, and television screens

Forty years of carabid beetle research in Europe - from taxonomy, biology, ecology and population studies to bioindication, habitat assessment and conservation

Volume: 100Start Page: 55End Page: 14

Correlation of the Isotropic NMR Chemical Shift with Oxygen Coordination Distances in Periodic Solids

In Nuclear Magnetic Resonance (NMR) spectroscopy, the isotropic chemical shift δiso is a measure of the electron density around the observed nuclide. For characterization of solid materials and compounds, it is desirable to find correlations between δiso and structural parameters such as coordination numbers and distances to neighboring atoms. Correlations of good quality are easier to find when the coordination sphere is formed by only one element, as the electron density is obviously strongly dependent on the atomic number. The current study is therefore restricted to nuclides in pure oxygen coordination. It is shown that the isotropic shift δiso correlates well with the average oxygen distances (as defined by the coordination sphere) for the nuclides 23Na (with spin I=3/2), 27Al (I=5/2), and 43Ca (I=7/2), using literature data for a range of periodic solids. It has been previously suggested for 207Pb (I=1/2) that δiso may alternatively be related to the shortest oxygen distance in the structure, and our study corroborates this also for the nuclides considered here. While the correlation with the minimal distance is not always better, it has the advantage of being uniquely defined. In contrast, the average distance is strongly dependent on the designation of the oxygen coordination sphere, which may be contentious in some crystal structures

Correlation of the Isotropic NMR Chemical Shift with Oxygen Coordination Distances in Periodic Solids

In Nuclear Magnetic Resonance (NMR) spectroscopy, the isotropic chemical shift δiso is a measure of the electron density around the observed nuclide. For characterization of solid materials and compounds, it is desirable to find correlations between δiso and structural parameters such as coordination numbers and distances to neighboring atoms. Correlations of good quality are easier to find when the coordination sphere is formed by only one element, as the electron density is obviously strongly dependent on the atomic number. The current study is therefore restricted to nuclides in pure oxygen coordination. It is shown that the isotropic shift δiso correlates well with the average oxygen distances (as defined by the coordination sphere) for the nuclides 23Na (with spin I=3/2), 27Al (I=5/2), and 43Ca (I=7/2), using literature data for a range of periodic solids. It has been previously suggested for 207Pb (I=1/2) that δiso may alternatively be related to the shortest oxygen distance in the structure, and our study corroborates this also for the nuclides considered here. While the correlation with the minimal distance is not always better, it has the advantage of being uniquely defined. In contrast, the average distance is strongly dependent on the designation of the oxygen coordination sphere, which may be contentious in some crystal structures

Frequency-swept pulse sequences for 19F heteronuclear spin decoupling in solid state NMR

Heteronuclear spin decoupling pulse sequences in solid-state NMR have mostly been designed and applied for irradiating 1H as the abundant nucleus. Here, a systematic comparison of different methods for decoupling 19F in rigid organic solids is presented, with a special emphasis on the recently introduced frequency-swept sequences. An extensive series of NMR experiments at different MAS frequencies was conducted on fluorinated model compounds, in combination with large sets of numerical simulations. From both experiments and simulations it can be concluded that the frequency-swept sequences SWf- TPPM and SWf-SPINAL deliver better and more robust spin decoupling than the original sequences SPINAL and TPPM. Whereas the existence of a large chemical shift anisotropy and isotropic shift dispersion for 19F does compromise the decoupling efficiency, the relative performance hierarchy of the sequences remains unaffected. Therefore, in the context of rigid organic solids under moderate MAS frequencies, the performance trends observed for 19F decoupling are very similar to those observed for 1H decouplin

Efficient 5QMAS NMR of spin-5/2 nuclei: use of fast amplitude-modulated radio-frequency pulses and cogwheel phase cycling

We report here an efficient multiple-quantum magic-angle spinning (MQMAS) pulse sequence involving fast amplitude-modulated (FAM) radio-frequency pulses for excitation and conversion of five-quantum (5Q) coherences of spin-5/2 nuclei. The use of a FAM-I type pulse train for the conversion of 5Q into 1Q coherences proves to be easier to implement experimentally than the earlier suggested use of a FAM-II type sequence [J. Magn. Reson. 154 (2002) 280], while delivering at least equal signal enhancement. Results of numerical simulations and experimental 27Al 5QMAS spectra of aluminium acetylacetonate for different excitation and conversion schemes are compared to substantiate these claims. We also demonstrate the feasibility of acquiring 5QMAS spectra of spin-5/2 systems using cogwheel phase cycling [J. Magn. Reson. 155 (2002) 300] to select the desired coherence pathways. A cogwheel phase cycle of only 57 steps is shown to be as effective as the minimum conventional nested 77-step phase cycle