Felix Fahrnbauer

Unexpected Luminescence Properties of Sr0.25Ba0.75Si2O2N2:Eu2+—A Narrow Blue Emitting Oxonitridosilicate with Cation Ordering

Owing to a parity allowed 4f(6)((7)F)5d(1)→4f(7)((8)S(7/2)) transition, powders of the nominal composition Sr(0.25)Ba(0.75)Si(2)O(2)N(2):Eu(2+) (2 mol% Eu(2+)) show surprising intense blue emission (λ(em)=472 nm) when excited by UV to blue radiation. Similarly to other phases in the system Sr(1-x)Ba(x)Si(2)O(2)N(2):Eu(2+), the described compound is a promising phosphor material for pc-LED applications as well. The FWHM of the emission band is 37 nm, representing the smallest value found for blue emitting (oxo)nitridosilicates so far. A combination of electron and X-ray diffraction methods was used to determine the crystal structure of Sr(0.25)Ba(0.75)Si(2)O(2)N(2):Eu(2+). HRTEM images reveal the intergrowth of nanodomains with SrSi(2)O(2)N(2) and BaSi(2)O(2)N(2)-type structures, which leads to pronounced diffuse scattering. Taking into account the intergrowth, the structure of the BaSi(2)O(2)N(2)-type domains was refined on single-crystal diffraction data. In contrast to coplanar metal atom layers which are located between layers of condensed SiON(3)-tetrahedra in pure BaSi(2)O(2)N(2), in Sr(0.25)Ba(0.75)Si(2)O(2)N(2):Eu(2+) corrugated metal atom layers occur. HRTEM image simulations indicate cation ordering in the final structure model, which, in combination with the corrugated metal atom layers, explains the unexpected and excellent luminescence properties.

Determination of the distribution of elements with similar electron counts: a practical guide for resonant X‐ray scattering

This article attempts to present straightforward and easy-to-understand guidelines for the determination of element distribution in compounds lacking X-ray scattering contrast because they have similar electron counts. Different sources of anomalous dispersion correction terms (especially Δf′ values) are compared with respect to their suitability, reliability and quality. Values from databases are compared with Δf values calculated from fluorescence spectra and those refined from single-crystal diffraction data, using both reference crystals without scattering contrast problems and crystals containing elements with similar electron counts. The number of data sets required to determine reliably the element distribution and the optimum wavelengths to be used are discussed. Joint multiple data set refinements are suitable for the refinement of multiply mixed occupancies of elements lacking scattering contrast. The most straightforward method of obtaining Δf′ values depends on the complexity of the problem to be solved and the precision required.

Inter-kingdom signaling by the Legionella Quorum Sensing Molecule LAI-1 modulates cell migration through an IQGAP1-Cdc42-ARHGEF9-dependent pathway

Small molecule signaling promotes the communication between bacteria as well as between bacteria and eukaryotes. The opportunistic pathogenic bacterium Legionella pneumophila employs LAI-1 (3-hydroxypentadecane-4-one) for bacterial cell-cell communication. LAI-1 is produced and detected by the Lqs (Legionella quorum sensing) system, which regulates a variety of processes including natural competence for DNA uptake and pathogen-host cell interactions. In this study, we analyze the role of LAI-1 in inter-kingdom signaling. L. pneumophila lacking the autoinducer synthase LqsA no longer impeded the migration of infected cells, and the defect was complemented by plasmid-borne lqsA. Synthetic LAI-1 dose-dependently inhibited cell migration, without affecting bacterial uptake or cytotoxicity. The forward migration index but not the velocity of LAI-1-treated cells was reduced, and the cell cytoskeleton appeared destabilized. LAI-1-dependent inhibition of cell migration involved the scaffold protein IQGAP1, the small GTPase Cdc42 as well as the Cdc42-specific guanine nucleotide exchange factor ARHGEF9, but not other modulators of Cdc42, or RhoA, Rac1 or Ran GTPase. Upon treatment with LAI-1, Cdc42 was inactivated and IQGAP1 redistributed to the cell cortex regardless of whether Cdc42 was present or not. Furthermore, LAI-1 reversed the inhibition of cell migration by L. pneumophila, suggesting that the compound and the bacteria antagonistically target host signaling pathway(s). Collectively, the results indicate that the L. pneumophila quorum sensing compound LAI-1 modulates migration of eukaryotic cells through a signaling pathway involving IQGAP1, Cdc42 and ARHGEF9.

La6Ba3[Si17N29O2]Cl—An Oxonitridosilicate Chloride with Exceptional Structural Motifs

The oxonitridosilicate chloride La6Ba3[Si17N29O2]Cl was synthesized by a high-temperature reaction in a radiofrequency furnace starting from LaCl3, BaH2, and the ammonolysis product of Si2Cl6. Diffraction data of a micrometer-sized single crystal were obtained using microfocused synchrotron radiation at beamline ID11 of the ESRF. EDX measurements on the same crystal confirm the chemical composition. The crystal structure [space group P63/m (no. 176), a = 9.8117(14), c = 19.286(6) Å, Z = 2] contains an unprecedented interrupted three-dimensional network of vertex-sharing SiN4 and SiN3O tetrahedra. The SiN4 tetrahedra form dreier rings. Twenty of the latter condense in a way that the Si atoms form icosahedra. Each icosahedron is connected to others via six SiN4 tetrahedra that are part of dreier rings and via six Q(3)-type SiN3O tetrahedra. Rietveld refinements confirm that the final product contains only a small amount of impurities. Lattice energy (MAPLE) and bond-valence sum (BVS) calculations show that the structure is electrostatically well balanced. Infrared spectroscopy confirms the absence of N-H bonds.

Highly Symmetric AB2 Framework Related to Tridymite in the Disordered Nitridosilicate La24Sr14–7x[Si36N72](O1–xFx)14 (x = 0.489)

La24Sr14-7x[Si36N72](O1-xFx)14 with x = 0.489 was obtained as a microcrystalline product by metathesis at 1500 °C in a radio-frequency furnace starting from Si(NH)2, La(NH2)3, SrH2, LaF3, and CeF3. The structure of the new nitridosilicate oxide fluoride was determined by combining transmission electron microscopy (TEM) and single-crystal X-ray diffraction using a microfocused synchrotron beam. The structure model with pronounced disorder [P63/mmc, Z = 1, a = 16.2065(3), c = 9.4165(1) Å, R1(obs) = 0.0436] was confirmed by electron diffraction and aberration-corrected Z-contrast scanning TEM. The highly symmetric AB2 framework, which was theoretically predicted but not yet realized, consists of all-side vertex-sharing SiN4 tetrahedra that form channels along [001] filled with La, Sr, O, and F atoms. The connectivity pattern is related to that of tridymite. X-ray spectroscopy and bond-valence-sum calculations were further taken into account for assignment of the N, O, and F atoms.

Unusual crystal structures in the system (SnSe)nBi2Se3

As intermediates between layered tellurides and chain-like sulfides, selenides exhibit a huge variety of structures and are promising candidates for thermoelectrics. The compounds (SnSe)nBi2Se3 cover the whole range of compound classes. (SnSe)0.5Bi2Se3 exhibits the 21R-GeSb2Te4 structure type (R-3m, a = 4.172Å, c = 38.86 Å, R1(obs) = 0.0246) [1] instead of the 12P stacking expected for this stoichiometry, HRTEM indicates the presence of stacking disorder. This phase coexists with “phase X” [2]. From such samples, e.g. (SnSe)~2Bi2Se3, single crystals of new compounds could be retrieved. Their structures resemble those of minerals of the lillianite series. Slowly cooled samples near the dystectic composition [2] in the phase diagram yielded lillianite-type (SnxBi1-x)5Se6 (x ≈ 2, Cmcm, a = 4.196 Å, b = 13.83 Å, c = 21.19 Å, R1(obs) = 0.0347) mixed with (SnxBi1-x)11Se13 (x ≈ 3, C2/m, a = 13.85 Å, b = 4.205 Å, c = 23.33 Å, β = 98.7°, R1(obs) = 0.0594), which is isotypic to KSn5Bi5Se13 [3]. Further a new phase (SnxBi1-x)8Se9 with heyrovskite-type structure (x ≈ 3, Cmcm, a = 4.193 Å, b = 13.87Å, c = 32.01 Å, R1(obs) = 0.0392) was obtained. These new structures consist of tropochemically twinned slabs that represent distorted cutouts of the rocksalt type. They are interconnected via cations in trigonal prisms. They are members of a homologous series that differ concerning the thickness of the slabs. The cubic high-temperature phase (SnSe)nBi2Se3 (~2.5 < n < 4) yields a metastable pseudocubic disordered rocksalt-type material (Fm-3m, a = 5.936 Å, R1(obs) = 0.0473) upon quenching. For n = 4, it contains homogeneously distributed nanoscale SnSe precipitates. Such a heterostructure might explain the low thermal conductivity (0.83 Wm-1K-1 at room temperature). The rather low electrical conductivity might be further tuned by substitution, which promises an intriguing approach for new nanostructured thermoelectrics easily accessible by partial phase transitions and exsolution effects.

Combining TEM and synchrotron microdiffraction – a novel lead antimony sulfide telluride, and more examples

Electron microscopy and electron diffraction in combination with EDX spectroscopy are excellent tools for discovering new compounds that are minority phases in complex reaction products; this is an intriguing task when metastable phases in quenched samples are important to understand reaction pathways. However, the precise determination of the crystal structure is often not straight-forward by electron crystallography. Therefore, crystallites were investigated by SAED and EDX to ensure that the combination of unit cell and composition proves the novelty. The preselected crystallites have been transferred to ID11’s (ESRF, Grenoble) micofocus beamline and localized on TEM grids by fluorescence scans which also made a reliable centering possible. “Normal” single-crystal X-ray datasets were collected that allow us to dis¬cuss the precise crystal structures in detail. Quenching Pb/Sb/S/Te melts affords Pb8Sb8S15Te5 (P41, a = 8.003 Å, c = 15.02 Å, Z = 1, R1 = 0.043) as a minority phase whose complicated Tl3PbCl5-type [1] structure has been determined by the method outlined. As the ionic radii of S and Te differ by more than 20%, mixed crystals are exceptional and expected to be metastable at room temperature. The structure is not related with those of typical compounds in the systems Pb/Sb/S (chain structures) or Pb/ Sb/Te (layer structures) but characterized by interconnected heterocubane-like entities. The cations, whose coordination spheres are irregular with CN = 8, form a sequence of nets built up from distorted squares and rhombi. Pb/Sb and S/Te are statistically distributed on the cation and anion positions, respectively. Preferred occupancies are reflected in the averaged bond lengths and can be understood by bond valence calculations. Pseu-dosymmetry leads to twinning around [110]. The structure was confirmed by HRTEM image simulations, which highlight the synergism between electron and synchrotron methods. The method was further applied to various layer structures derived from Ge/Sb/Te materials which are interesting because of their thermoelectric properties and to other classes of materials such as oxonitridophosphates.

Diffusion processes in (GeTe)n(Sb2Te3) (4 < n < 12) crystals investigated by synchrotron radiation

crystals investigated by synchrotron radiation. Philipp Urban, Matthias N. Schneider, Jonathan Wright, Xavier Biquard, Simon Welzmiller, Felix Fahrnbauer, Oliver Oeckler, Leipzig University, IMKM, Scharnhorststr. 20, 04275 Leipzig, Germany, LMU Munich, Department of Chemistry, Butenandtstr. 5-13, 81377 Munich, Germany, ESRF, 6 rue Jules Horowitz, 38000 Grenoble, France. E-mail: philipp.urban@uni-leipzig.de

Thermoelectric properties of metastable Ge/Sb/Te and Ge/Bi/Te compounds

For both (GeTe)n(Sb2Te3) (n = 3, 4.5, 7, 12, 19) and GeBi2Te4, samples with different nanostructures can be obtained. The nanostructure of (GeTe)n(Sb2Te3) depends on the GeTe content n as well as on the thermal treatment of the samples. Low GeTe contents (n < 4.5) lead to a high concentration of vacancies, which arrange in parallel defect layers and form van der Waals gaps upon annealing. Higher GeTe contents, i.e. less defects, lead to pseudocubic structures with finite intersecting vacancy planes. Annealing such samples at 450 °C leads to diffusion, yielding parallel gaps between distorted rocksalt-type blocks. ZT values of up to 1.3 have been obtained for quenched samples with n = 12 or 19, whereas the maximum ZT = 0.3 was reached for annealed samples with parallel van der Waals gaps. For GeBi2Te4, various nanostructures can be introduced by crystallization under high pressure (12 GPa) and different thermal treatment. Small particles (< 10 nm) with a high concentration of grain boundaries can be obtain...