Missing Member in the MII MIII Si4 N7 Compound Class: Carbothermal Reduction and Nitridation Synthesis Reveal Substitution of Nitrogen by Carbon and Oxygen in CaLu[Si4 N7-2x Cx Ox ]:Eu2+ /Ce3+ (x≈0.3).

The nitridosilicate CaLu[Si4 N7-2x Cx Ox ] (x≈0.3) was synthesized by carbothermal reduction and nitridation starting from CaH2 , Lu2 O3 , graphite and amorphous Si3 N4 at 1550 °C in a radiofrequency furnace. CaLu[Si4 N7-2x Cx Ox ] (x≈0.3) crystallizes isotypically to many previously known MII MIII Si4 N7 compounds in the space group P63 mc, as was confirmed by Rietveld refinement based on powder X-ray diffraction data. Incorporation of carbon into the crystal structure as a result of the carbothermal synthesis route was confirmed by 13 C and 29 Si MAS NMR spectroscopy. For the first time in the MII MIII Si4 N7 compound class, complementary EDX measurements suggest that simultaneous incorporation of oxygen compensates for the negative charge excess induced by carbon, resulting in an adjusted sum formula, CaLu[Si4 N7-2x Cx Ox ] (x≈0.3). When excited with UV-to-blue light, CaLu[Si4 N7-2x Cx Ox ] (x≈0.3) shows an emission maximum in the blue spectral region (λem =484 nm; fwhm=4531 cm-1 ) upon doping with Ce3+ , whereas Eu2+ -doped CaLu[Si4 N7-2x Cx Ox ] (x≈0.3) exhibits a yellow-green emission (λem =546 nm; fwhm=3999 cm-1 ).

Quantifying the quadrupolar interaction by 45Sc-NMR spectroscopy of single crystals.

Single crystals of the compound [ [Formula: see text] ]Cl4 ⋅ 2H2O were studied by 45Sc-NMR, with the effect of the quadrupolar coupling interaction on the spectra of the spin-7/2 nucleus analysed in the hierarchical framework of perturbation theory. Orientation-dependent spectra acquired at B0 = 17.6 T showed strong second-order effects due to the comparatively large coupling constant of χ = |14.613 ±â€¯0.006| MHz, with an associated asymmetry parameter of ηQ = 0.540 9 ±â€¯0.000 4. By analysing the splittings of the ±3/2 satellites, which in good approximation are subjected to first-order effects only, the full quadrupolar coupling tensor could be determined. The second-order effects caused by this tensor were calculated according to theoretical predictions for all orientations, and subtracted from both the centres of gravity of the satellites, and the central transitions. This allowed extraction of the full chemical shift tensor, with the eigenvalues being δ11 = (5.6 ±â€¯0.9) ppm, δ22 = (12.4 ±â€¯0.9) ppm, and δ33 = (38.5 ±â€¯0.9) ppm. In spectra acquired at a lower magnetic field of B0 = 9.4 T, third-order effects could be detected, and similarly quantified using analytical expressions.

Polymorphism and Fast Potassium-Ion Conduction in the T5 Supertetrahedral Phosphidosilicate KSi2 P3.

The all-solid-state battery (ASSB) is a promising candidate for electrochemical energy storage. In view of the limited availability of lithium, however, alternative systems based on earth-abundant and inexpensive elements are urgently sought. Besides well-studied sodium compounds, potassium-based systems offer the advantage of low cost and a large electrochemical window, but are hardly explored. Here we report the synthesis and crystal structure of K-ion conducting T5 KSi2 P3 inspired by recent discoveries of fast ion conductors in alkaline phosphidosilicates. KSi2 P3 is composed of SiP4 tetrahedra forming interpenetrating networks of large T5 supertetrahedra. The compound passes through a reconstructive phase transition from the known T3 to the new tetragonal T5 polymorph at 1020 °C with enantiotropic displacive phase transitions upon cooling at about 155 °C and 80 °C. The potassium ions are located in large channels between the T5 supertetrahedral networks and show facile movement through the structure. The bulk ionic conductivity is up to 2.6×10-4  S cm-1 at 25 °C with an average activation energy of 0.20 eV. This is remarkably high for a potassium ion conductor at room temperature, and marks KSi2 P3 as the first non-oxide solid potassium ion conductor.

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 ( 14 N ) = ( 8 . 19 ± 0 . 02 ) kHz, and χ ( 27 Al ) = ( 1 . 914 ± 0 . 001 ) MHz. The chemical shift parameters obtained from the data fit were δ i s o = - ( 292 . 6 ± 0 . 6 ) ppm and δ Δ = - ( 1 . 9 ± 1 . 1 ) ppm for 14 N, and (after correcting for the second-order quadrupolar shift) δ i s o = ( 113 . 6 ± 0 . 3 ) ppm and δ Δ = ( 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.

Single-crystal 207Pb-NMR of wulfenite, PbMoO4, aided by simultaneous measurement of phosgenite, Pb2Cl2CO3.

The effort for determining NMR interaction tensors from orientation-dependent spectra of single crystals may be greatly reduced by exploiting symmetry relations between atoms of the observed nuclide in the unit cell, as is well documented in the literature. In this work, we determined both the full chemical shift (CS) tensor of 207Pb and the unknown orientation of the rotation axis for the natural mineral phosgenite, Pb2Cl2CO3, from a single rotation pattern, i.e. spectra of crystal orientations from 0 to 180°. In the tetragonal crystal structure of phosgenite, four symmetry-related, but magnetically inequivalent 207Pb are generated by the Wyckoff multiplicity. The mineral wulfenite, PbMoO4, also crystallises in a tetragonal space group, but the site multiplicity for 207Pb generates only one magnetically inequivalent atom, thus not supplying sufficient experimental data to determine CS tensor and axis orientation from an arbitrary number of rotation patterns. One solution to this problem is to simultaneously acquire data of a known compound with high symmetry and Wyckoff multiplicity (here: phosgenite), which supplies additional constraints making the solution of the target compound (here: wulfenite) possible. The 207Pb CS tensors thus determined are characterised by the following eigenvalues in ppm: δ11PAS=(-2553±1), δ22PAS=(-1929±1), δ33PAS=(-1301±1) for phosgenite, and δ11PAS=(-2074±1), δ22PAS=(-2074±1), δ33PAS=(-1898±1) for wulfenite.

NMR interaction tensors of 51V and 207Pb in vanadinite, Pb5(VO4)3Cl, determined from DFT calculations and single-crystal NMR measurements, using only one general rotation axis.

Orientation-dependent NMR spectra of a single crystal of the mineral vanadinite, Pb5(VO4)3Cl, were acquired using only one rotation axis with a general orientation in the hexagonal crystal lattice (space group P63/m). The chemical shift (CS) tensors for the 207Pb on Wyckoff positions 6h and 4f, and both CS and quadrupole coupling tensor Q for 51V at the positions 6h were determined by including the NMR response of symmetry-related atoms in the unit cell (and in case of 207Pb at 4f, also the isotropic shift from MAS NMR spectra). This previously suggested 'single rotation method' greatly reduces the necessary amount of data acquisition and analysis. The precise orientation of the rotation axis could not be found by X-ray diffraction experiments because of the high linear absorption coefficient of vanadinite, which is chiefly due to its high lead content. The axis orientation was therefore included into the multi-parameter data fit routine. This NMR-based approach is widely applicable, and offers an alternative way of orienting single crystals. The NMR parameters derived from the tensor eigenvalues are δiso=(-1729±9) ppm, Δδ=(-1071±5) ppm, ηCS=0.362±0.008 for 207Pb at positions 6h, and δiso=(-1619±2) ppm, Δδ=(-780±58) ppm, ηCS=0.06±0.08 for positions 4f. For 51V, δiso=(-509±3) ppm, Δδ=(-37±2) ppm, ηCS=0.78±0.09, with the quadrupolar coupling described by χ=(2.52±0.01) MHz and ηQ=0.047±0.003. In contrast to the precisely determined tensor eigenvalues, the orientation of the eigenvectors in the crystal ab -plane of the vanadinite system could only be resolved by resorting to data obtained from density functional theory (DFT) calculations.

Supertetrahedral anions in the phosphidosilicates Na1.25Ba0.875Si3P5 and Na31Ba5Si52P83.

Solid ionic conductors are one key component of all-solid-state batteries, and recent studies with lithium, sodium and potassium phosphidosilicates revealed remarkable ion conduction capabilities in these compounds. We report the synthesis and crystal structures of two quaternary phosphidosilicates with sodium and barium, which crystallize in new structure types. Na1.25Ba0.875Si3P5 contains layers of T3 supertetrahedra, while Na31Ba5Si52P83 forms defect T5 entities and contains Si-Si bonds and P3 trimers. Though T1-relaxometry data indicate a relatively low activation energy for Na+ migration of 0.16 eV, the crystal structures lack sufficient three-dimensional migration paths necessary for fast sodium ion conductvity.