Oliver Oeckler

Synthesis of a Möbius aromatic hydrocarbon.

The defining feature of aromatic hydrocarbon compounds is a cyclic molecular structure stabilized by the delocalization of π electrons that, according to the Hückel rule, need to total 4n + 2 (n = 1,2,…); cyclic compounds with 4n π electrons are antiaromatic and unstable. But in 1964, Heilbronner predicted on purely theoretical grounds that cyclic molecules with the topology of a Möbius band—a ring constructed by joining the ends of a rectangular strip after having given one end half a twist—should be aromatic if they contain 4n, rather than 4n + 2, π electrons. The prediction stimulated attempts to synthesize Möbius aromatic hydrocarbons, but twisted cyclic molecules are destabilized by large ring strains, with the twist also suppressing overlap of the p orbitals involved in electron delocalization and stabilization. In larger cyclic molecules, ring strain is less pronounced but the structures are very flexible and flip back to the less-strained Hückel topology. Although transition-state species, an unstable intermediate and a non-conjugated cyclic molecule, all with a Möbius topology, have been documented, a stable aromatic Möbius system has not yet been realized. Here we report that combining a ‘normal’ aromatic structure (with p orbitals orthogonal to the ring plane) and a ‘belt-like’ aromatic structure (with p orbitals within the ring plane) yields a Möbius compound stabilized by its extended π system.

Material Properties and Structural Characterization of M3Si6O12N2:Eu2+ (M=Ba, Sr)—A Comprehensive Study on a Promising Green Phosphor for pc‐LEDs

The efficient green phosphor Ba(3)Si(6)O(12)N(2):Eu(2+) and its solid-solution series Ba(3-x)Sr(x)Si(6)O(12)N(2) (with x approximately = 0.4 and 1) were synthesized in a radio-frequency furnace under nitrogen atmosphere at temperatures up to 1425 degrees C. The crystal structure (Ba(3)Si(6)O(12)N(2), space group P3 (no. 147), a = 7.5218(1), c = 6.4684(1) A, wR2 = 0.048, Z = 1) has been solved and refined on the basis of both single-crystal and powder X-ray diffraction data. Ba(3)Si(6)O(12)N(2):Eu(2+) is a layer-like oxonitridosilicate and consists of vertex-sharing SiO(3)N-tetrahedra forming 6er- and 4er-rings as fundamental building units (FBU). The nitrogen atoms are connected to three silicon atoms (N3), while the oxygen atoms are either terminally bound (O1) or bridge two silicon atoms (O2) (numbers in superscripted square brackets after atoms indicate the coordination number of the atom in question). Two crystallographically independent Ba(2+) sites are situated between the silicate layers. Luminescence investigations have shown that Ba(3)Si(6)O(12)N(2):Eu(2+) exhibits excellent luminescence properties (emission maximum at approximately 527 nm, full width at half maximum (FWHM) of approximately 65 nm, low thermal quenching), which provides potential for industrial application in phosphor-converted light-emitting diodes (pc-LEDs). In-situ high-pressure and high-temperature investigations with synchrotron X-ray diffraction indicate decomposition of Ba(3)Si(6)O(12)N(2) under these conditions. The band gap of Ba(3)Si(6)O(12)N(2):Eu(2+) was measured to be 7.05+/-0.25 eV by means of X-ray emission spectroscopy (XES) and X-ray absorption near edge spectroscopy (XANES). This agrees well with calculated band gap of 6.93 eV using the mBJ-GGA potential. Bonding to the Ba atoms is highly ionic with only the 4p(3/2) orbitals participating in covalent bonds. The valence band consists primarily of N and O p states and the conduction band contains primarily Ba d and f states with a small contribution from the N and O p states.

Sr5Al5+xSi21−xN35−xO2+x:Eu2+ (x≈0)—A Novel Green Phosphor for White‐Light pcLEDs with Disordered Intergrowth Structure

Sr(5)Al(5+x)Si(21-x)N(35-x)O(2+x) (x approximately 0) was obtained by high-temperature synthesis (1600 to 1750 degrees C). Upon doping with Eu(2+), the thermally very stable material shows an efficient broadband emission in the green spectral range (lambda(max) approximately 510 nm, FWHM = 69 nm) under UV to blue light excitation. The compound exhibits a complex intergrowth structure (space group Pmn2(1) (no. 31); a = 23.614, b = 7.487, c = 9.059 A; V = 1601.5(6) A(3); Z = 2, R1 = 0.067), which consists of highly condensed dreier ring layers alternating with sechser ring layers that include both vertex- and edge-sharing (Si,Al)(O,N)(4) tetrahedra. Both layer types exhibit pseudotranslational symmetry, which leads to a more or less pronounced disorder of the sechser ring layers. The Sr atoms are located in channel-like voids of the silicate framework with coordination number nine. The compound has been characterized by single-crystal and powder X-ray diffraction, as well as high-resolution electron microscopy and electron diffraction. The structure and chemical composition has been confirmed by (29)Si solid-state NMR spectroscopy, lattice energy calculations, and diverse elemental analyses.

From phase-change materials to thermoelectrics?

Abstract Metastable tellurides play an important role as phase-change materials in data storage media and non-vol atile RAM devices. The corresponding crystalline phases with very simple basic structures are not stable as bulk materials at ambient conditions, however, for a broad range of compositions they represent stable high-temperature phases. In the system Ge/Sb/Te, rocksalt-type high-temperature phases are characterized by a large number of vacancies randomly distributed over the cation position, which order as 2D vacancy layers upon cooling. Short-range order in quenched samples produces pronounced nanostructures by the formation of twin domains and finite intersecting vacancy layers. As phase-change materials are usually semimetals or small-bandgap semiconductors and efficient data storage requires low thermal conductivity, bulk materials with similar compositions and properties can be expected to exhibit promising thermoelectric characteristics. Nanostructuring by phase transitions that involve partial vacany ordering may enhance the efficiency of such thermoelectrics. We have shown that germanium antimony tellurides with compositions close to those used as phase-change materials in rewritable Blu-Ray Discs, e.g. (GeTe)12Sb2Te3, exhibit thermoelectric figures of merit of up to ZT = 1.3 at 450 °C if a nanodomain structure is induced by rapidly quenching the cubic high-temperature phase. Structural changes have been elucidated by X-ray diffraction and high-resolution electron microscopy.

Unprecedented Zeolite-Like Framework Topology Constructed from Cages with 3-Rings in a Barium Oxonitridophosphate

A novel oxonitridophosphate, Ba(19)P(36)O(6+x)N(66-x)Cl(8+x) (x ≈ 4.54), has been synthesized by heating a multicomponent reactant mixture consisting of phosphoryl triamide OP(NH(2))(3), thiophosphoryl triamide SP(NH(2))(3), BaS, and NH(4)Cl enclosed in an evacuated and sealed silica glass ampule up to 750 °C. Despite the presence of side phases, the crystal structure was elucidated ab initio from high-resolution synchrotron powder diffraction data (λ = 39.998 pm) applying the charge flipping algorithm supported by independent symmetry information derived from electron diffraction (ED) and scanning transmission electron microscopy (STEM). The compound crystallizes in the cubic space group Fm ̅3c (no. 226) with a = 2685.41(3) pm and Z = 8. As confirmed by Rietveld refinement, the structure comprises all-side vertex sharing P(O,N)(4) tetrahedra forming slightly distorted 3(8)4(6)8(12) cages representing a novel composite building unit (CBU). Interlinked through their 4-rings and additional 3-rings, the cages build up a 3D network with a framework density FD = 14.87 T/1000 Å(3) and a 3D 8-ring channel system. Ba(2+) and Cl(-) as extra-framework ions are located within the cages and channels of the framework. The structural model is corroborated by (31)P double-quantum (DQ) /single-quantum (SQ) and triple-quantum (TQ) /single-quantum (SQ) 2D correlation MAS NMR spectroscopy. According to (31)P{(1)H} C-REDOR NMR measurements, the H content is less than one H atom per unit cell.

Highly Efficient pc-LED Phosphors Sr1-xBaxSi2O2N2:Eu2+ (0 x 1) - Crystal Structures and Luminescence Properties Revisited

Due to their excellent luminescence properties in the blue-green to green-yellow spectral region, oxonitridosilicates Sr1-xBaxSi2O2N2:Eu2+ (0 x 1) are promising conversion materials for application in phosphor-converted high-power LED devices. In order to understand the properties and thus to fully exploit the potential of these materials, detailed knowledge of corresponding (local) crystal structures is indispensable. Detailed insights into real structures have been achieved by combining X-ray diffraction and electron-microscopy methods. A major reason for the excellent luminescence properties of the phases Sr1-xBaxSi2O2N2:Eu2+ (0 x 1) is the rigid silicate substructure built up of two-dimensionally condensed SiON3 tetrahedra. The general topology of these layers is analogous for all members. However, there is no complete solid-solution series. Crystal-structure determination was frequently not straightforward because several real-structure effects had to be considered. The relative orientation of the silicate layers and the metal-atom layers inserted between them can differ without changing the chemical composition. As a consequence, polytypes are formed. The differentiation between such closely related structures was only possible by a thorough analysis of crystallographic data. The same applies for phases which differ in their composition as all Sr1-xBaxSi2O2N2:Eu2+ (0 x 1) phases are very similar. The literature on these compounds is critically discussed with respect to phase analysis and structure determination. Different synthesis routes are reviewed and the results of luminescence investigations are discussed in this contribution. Beyond thermal as well as chemical stability and high transparency, electron-phonon coupling is effectively suppressed in Sr1-xBaxSi2O2N2:Eu2+ (0 x 1) phases. Therefore, primary UV to blue light (GaN based semiconductor LEDs) is efficiently converted into visible components of the spectrum. Sr1-xBaxSi2O2N2:Eu2+ (0 x 1) phases are therefore promising oxonitridosilicate phosphors for application in LED industry.

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.

B and B–C as interstitials in reduced rare earth halides

Abstract In this review we describe a variety of rare earth metal boride halides and boride carbide halides. In boride halides boron atoms occur either as discrete atoms octahedrally coordinated by rare earth metal atoms or as chains of interconnected B 4 rhomboids in rods of fused metal atom bisphenoids. Characteristic building units in boride carbide halides are quasi-molecular B–C, C–B–C, C–B–B–C, and C 2 –B–B–C 2 entities. In these compounds B centers trigonal prisms of rare earth metal atoms, and C is located in tetragonal pyramids. Owing to the electropositive character of rare earth metals, B atoms, B 4 units, and C n B m groups tend to be anions. Band structure calculations as well as electrical and magnetic measurements have been performed. The compounds show semiconducting or metallic behavior, depending on whether excess electrons according to the Zintl–Klemm formalism are localized or delocalized. La 9 Br 5 (CBC) 3 is superconducting with T c ∼6 K. Ce and Gd compounds are paramagnetic. In most cases anti-ferromagnetic ordering is observed at low temperature.

Correlation of magnetoelectric coupling in multiferroic BaTiO3-BiFeO3 superlattices with oxygen vacancies and antiphase octahedral rotations

Multiferroic (BaTiO3-BiFeO3) × 15 multilayer heterostructures show high magnetoelectric (ME) coefficients αME up to 24 V/cm·Oe at 300 K. This value is much higher than that of a single-phase BiFeO3 reference film (αME = 4.2 V/cm·Oe). We found clear correlation of ME coefficients with increasing oxygen partial pressure during growth. ME coupling is highest for lower density of oxygen vacancy-related defects. Detailed scanning transmission electron microscopy and selected area electron diffraction microstructural investigations at 300 K revealed antiphase rotations of the oxygen octahedra in the BaTiO3 single layers, which are an additional correlated defect structure of the multilayers.

HP-Ca2Si5N8—A New High-Pressure Nitridosilicate: Synthesis, Structure, Luminescence, and DFT Calculations

HP-Ca2Si5N8 was obtained by means of high-pressure high-temperature synthesis utilizing the multianvil technique (6 to 12 GPa, 900 to 1200 degrees C) starting from the ambient-pressure phase Ca2Si5N8. HP-Ca2Si5N8 crystallizes in the orthorhombic crystal system (Pbca (no. 61), a=1058.4(2), b=965.2(2), c=1366.3(3) pm, V=1395.7(7)x10(6) pm3, Z=8, R1=0.1191). The HP-Ca2Si5N8 structure is built up by a three-dimensional, highly condensed nitridosilicate framework with N[2] as well as N[3] bridging. Corrugated layers of corner-sharing SiN4 tetrahedra are interconnected by further SiN4 units. The Ca2+ ions are situated between these layers with coordination numbers 6+1 and 7+1, respectively. HP-Ca2Si5N8 as well as hypothetical orthorhombic o-Ca2Si5N8 (isostructural to the ambient-pressure modifications of Sr2Si5N8 and Ba2Si5N8) were studied as high-pressure phases of Ca2Si5N8 up to 100 GPa by using density functional calculations. The transition pressure into HP-Ca2Si5N8 was calculated to 1.7 GPa, whereas o-Ca2Si5N8 will not be adopted as a high-pressure phase. Two different decomposition pathways of Ca2Si5N8 (into Ca3N2 and Si3N4 or into CaSiN2 and Si3N4) and their pressure dependence were examined. It was found that a pressure-induced decomposition of Ca2Si5N8 into CaSiN2 and Si3N4 is preferred and that Ca2Si5N8 is no longer thermodynamically stable under pressures exceeding 15 GPa. Luminescence investigations (excitation at 365 nm) of HP-Ca2Si5N8:Eu2+ reveal a broadband emission peaking at 627 nm (FWHM=97 nm), similar to the ambient-pressure phase Ca2Si5N8:Eu2+.