New layered germanide halides RE2GeX2 (RE = Y, Gd; X = Br, I)

The title compounds were synthesized from RE, REX3, and Ge under an Ar atmosphere at 1200-1370 K. Y2GeI2 and Gd2GeI2 crystallize in space group Rm with lattice constants a = 4.2135(3) and 4.2527(1) A and c = 31.480(2) and 31.657(1) A, respectively. Gd2GeBr2 crystallizes in two modifications, the 1T-type (space group Pm1; a = 4.1668(2) A, c = 9.8173(6) A) and the 3R-type (space group Rm; a = 4.1442(9) A, c = 29.487(7) A). The structural motifs of RE2GeX2 compounds are Ge-centered slightly distorted RE6 octahedra connected via their common edges and extending in the a and b directions. The resulting close-packed double layers are separated by halogen atoms. The electrical resistivity measurements revealed semiconductor behavior for Y2GeI2 and Gd2GeI2 and a metal-semiconductor transition for 1T-Gd2GeBr2. Magnetic susceptibility and heat capacity measurements show long-range magnetic ordering for Gd2GeI2 and 1T-Gd2GeBr2 at approximately 15 and approximately 13 K, respectively.     

Crystal chemistry and physical properties of the ternary compounds REBC (RE = Ce,Pr, Nd)

The ternary rare-earth metal boride carbides REBC (RE = Ce, Pr, Nd) were prepared by melting mixtures of the elements and subsequent annealing at temperatures between 1270 K and 1570 K. Their crystal structures were refined from single crystal X-ray diffraction data. They crystallize in the LaBC-type structure (space group P2₁2₁2₁, Z = 20); CeBC: a = 8.5021(5) Å, b = 8.5217(7) Å, c = 12.3834(7) Å, R1 = 0.033 (wR2 = 0.059) for 2838 reflections with I_o > 2σ(I_o); PrBC: a = 8.4478(5) Å, b = 8.4719(8) Å, c = 12.325(1) Å, R1 = 0.031 (wR2 = 0.063) for 2564 reflections with I_o > 2σ(I_o); NdBC: a = 8.370(1) Å, b = 8.392(1) Å, c = 12.253(3) Å, R1 = 0.035 (wR2 = 0.086) for 4275 reflections with I_o > 2σ(I_o). The structure consists of a three-dimensional framework of rare-earth metal atoms resulting from the stacking of slightly corrugated two-dimensional square nets, leading to voids filled with B₅C₅ finite chains. The magnetism of the compounds PrBC and NdBC is characterized by the onset of ferromagnetism with Curie temperatures around 10 K and 8 K, respectively. The reduced effective paramagnetic moment μ_eff ≈ 1.8 μ_B as well as the weak magnetization at 6 K, 5 T is discussed.     

Asymptotic Results for the Sum of Dependent Non-identically Distributed Random Variables

In this paper we extend some results about the probability that the sum of n dependent subexponential random variables exceeds a given threshold u. In particular, the case of non-identically distributed and not necessarily positive random variables is investigated. Furthermore we establish criteria how far the tail of the marginal distribution of an individual summand may deviate from the others so that it still influences the asymptotic behavior of the sum. Finally we explicitly construct a dependence structure for which, even for regularly varying marginal distributions, no asymptotic limit of the tail of the sum exists. Some explicit calculations for diagonal copulas and t-copulas are given. Dominik Kortschak was supported by the Austrian Science Fund Project P18392.     

3D-PTV measurements in a plane Couette flow

Genuine plane Couette flow is hard to realize experimentally, and no applications of modern spatially resolving measurement techniques have been reported for this flow so far. In order to resolve this shortcoming, we designed and built a new experimental facility and present our first results here. Our setup enables us to access the flow via 3D particle tracking velocimetry and therefore to obtain truly three-dimensional flow fields for the first time experimentally in plane Couette flow. Results are analyzed in terms of basic flow properties, and a clear distinction of flow regimes (laminar for Re < 320, transitional for 320 < Re < 400, and turbulent when Re > 400) could be made. Comparison with DNS data shows good agreement in the turbulent regime and builds trust in our data. Furthermore, vortical coherent structures are studied in detail with the additional help of kalliroscope imaging, and the typical vortex spacing is determined to be roughly one gap width. As a noteworthy result, we find that the onset of the turbulent regime coincides with the range of Reynolds numbers at which a distance of 100 wall units is comparable to the gap width.     

Synthesis and Characterization of Terminal [Re(XCO)(CO)2(triphos)] (X=N,P): Isocyanate versus Phosphaethynolate Complexes

The terminal rhenium(I) phosphaethynolate complex [Re(PCO)(CO)₂(triphos)] has been prepared in a salt metathesis reaction from Na(OCP) and [Re(OTf)(CO)₂(triphos)]. The analogous isocyanato complex [Re(NCO)(CO)₂(triphos)] has been likewise prepared for comparison. The structure of both complexes was elucidated by X‐ray diffraction studies. While the isocyanato complex is linear, the phosphaethynolate complex is strongly bent around the pnictogen center. Computations including natural bond orbital (NBO) theory, natural resonance theory (NRT), and natural population analysis (NPA) indicate that the isocyanato complex can be viewed as a classic Werner‐type complex, that is, with an electrostatic interaction between the Re^I and the NCO group. The phosphaethynolate complex [Re(P?C?O)(CO)₂(triphos)] is best described as a metallaphosphaketene with a Re^I–phosphorus bond of highly covalent character.     

Testing the Universality of Free Fall for Charged Particles in Space

At first a short analysis of the notion of the Universality of Free Fall (UFF) for charged matter is given. Even if neutral bound systems of charged particles are in full accordance with the UFF, there is still a possibility that an isolated charge couples anomalously to gravitational fields. The experiment of Witteborn and Fairbank aimed at testing the UFF for electrons is shortly reviewed emphasizing the various additional disturbing gravity induced electromagnetic fields. Since these additional gravity induced fields are not very well under control, a space borne version of this experiment will reduce these disturbances considerably. The corresponding estimates for these kinds of tests in space are presented. As a result, gravity–induced stray field can be reduced considerably. Furthermore, also patch–effects can be reduced efficiently due to novel coating techniques. Therefore, due to microgravity conditions and new techniques the UFF for charged particles may be tested with much higher accuracy than in previous experiments.     

L3C3P3: Tricarbontriphosphide Tricyclic Radicals and Cations Stabilized by Cyclic (alkyl)(amino)carbenes

Alkynes usually oligomerize to give rings with a conjugated π‐electron system. In contrast, phosphaalkynes, R?C≡P, frequently give compounds with polycyclic structures, which are thermodynamically more stable than the corresponding π‐conjugated isomers. The syntheses of the first C₃P₃ tricyclic compounds are reported with either radical or cationic ground states stabilized by cyclic (alkyl)(amino)carbenes (CAACs). These compounds may be considered as examples of tricarbontriphosphide coordinated by carbenes and are likely formed via trimerization of the corresponding mono‐radicals CAAC‐CP^.. The mechanism for the formation of these tricarbontriphosphide radicals has been rationalized by a combination of experiments and DFT calculations.     

Amine olefin rhodium(I) complexes: pKa and NH bond strength

A combination of the rigid bis(5H-dibenzo[a,d]cyclohepten-5-yl)amine (trop(2)NH) and 5-amino-5H-dibenzo[a,d]cycloheptene (tropNH(2)) ligand allowed the synthesis of the stable pentacoordinated 18 electron amine olefin rhodium(i) complex [Rh(trop(2)NH)(tropNH(2))]O(3)SCF(3)(); this complex can be cleanly deprotonated [pK(a)(DMSO) = 20.6(1)] to the corresponding amide [Rh(trop(2)N)(tropNH(2))](6) which is reversibly oxidised at -0.466 V (vs. Fc/Fc(+)). The coordinated NH bond strength in is estimated to be 379 +/- 10 kJ mol(-1).