Calcium-free Solid Solutions in the System Ba7F12Cl2−xBrx (x<1.5), a Single-component White Phosphor Host

We have recently prepared solid solutions of Ba∼6.3Ca∼0.7F12Cl2−xBrx with x ranging from 0 to 2. In this work, the synthesis and single crystal X-ray structure of calcium-free crystals of Ba∼6.9Na∼0.2F12Br0.6Cl1.4 (space group P63/m, a=10.6024(10), c=4.2034(4)Å), Ba∼6.9Na∼0.2F12Br1.4 Cl0.6 (space group P63/m, a=10.6155(9), c=4.2355(4)Å) and Ba∼6.9Na∼0.2Br1.32Cl0.68F12 (space group P63/m, a=10.6218(9), c=4.2284(4)Å) are reported. These crystals systematically present additional electron density at the 0 0 0.25 position which is associated with the presence of small, but significant amounts of Na+ ions in the crysta

Synthesis and Structure of Ba

Abstract. Crystals of ordered and disordered Ba 7 F 12 Cl 2 were prepared by flux growth and solid state reactions. These new structures were characterized by single crystal and powder X-ray diffraction. The disordered variant which shows disorder on one of the cation sites was obtained from a BaF 2 + BaCl 2 + NaCl/NaF flux. It has hexagonal space group P6 3 /m (176) with one formula unit per unit cell. The lattice constants are a = b = 1059.55(5) pm and c = 420.10(4) pm (at 21°C). The structure was refined to R(R w ) = 0.026(0.030) for 346 independent reflections and 26 parameters. Slow cooling of a mixture of BaF 2 and LiCl yields the ordered variant. This one crystallizes in the hexagonal space group P6 (174) with one formula per unit cell. Lattice constants at 21°C are a = b = 1063.46(2) pm and c = 417.52(1) pm. The structure was refined to R(R w ) = 0.017 (0.017) for 638 independent reflections and 45 parameters. The structural arrangement and the interatomic distances of the two variants are mutually similar. The barium atoms have coordination number nine. Propeller-type arrangements with a chloride ion on the axis and the fluoride ions as blades are observed. These latter ones are interconnected into`channels&apos; of tricapped fluoride prisms. Occupation disorder of the barium sites in the channels of the disordered variant makes the main difference between the two. An unexpectedly high X-ray density obtained for both variants of Ba 7 F 12 Cl 2 can be correlated to the density of other barium fluorohalides having a coordination number of nine for the barium ion. Der strukturelle Aufbau und die atomaren Absta È nde beider Varianten sind anna È hernd gleich. Die Bariumatome haben die Koordinationszahl neun. Es werden propellerartige Anordnungen mit einem Chloridion in der Achse und den Fluoridionen als Flu È gel beobachtet. Die letzteren sind durch Kana È le von dreifach bekappten Fluoridprismen verbunden. Die Besetzung der fehlgeordneten Bariumlagen in den Kana È len der Fehlordnungsvariante ist der wesentliche Unterschied zwischen beiden Varianten. Die unerwartet hohe Ro È ntgendichte fu È r beide Formen von Ba 7 F 12 Cl 2 entspricht der Dichte anderer Bariumfluoridhalogenide mit der Koordinationszahl 9 fu È r das Bariumatom

Quantifying Morphology and Diffusion Properties of Mesoporous Carbon from High-Fidelity 3D Reconstructions

A reliable quantitative analysis in electron tomography, which depends on the segmentation of the three-dimensional reconstruction, is challenging because of constraints during tilt-series acquisition (missing wedge) and reconstruction artifacts introduced by reconstruction algorithms such as the Simultaneous Iterative Reconstruction Technique (SIRT) and Discrete Algebraic Reconstruction Technique (DART). We have carefully evaluated the fidelity of segmented reconstructions analyzing a disordered mesoporous carbon used as support in catalysis. Using experimental scanning transmission electron microscopy (STEM) tomography data as well as realistic phantoms, we have quantitatively analyzed the effect on the morphological description as well as on diffusion properties (based on a random-walk particle-tracking simulation) to understand the role of porosity in catalysis. The morphological description of the pore structure can be obtained reliably both using SIRT and DART reconstructions even in the presence of a limited missing wedge. However, the measured pore volume is sensitive to the threshold settings, which are difficult to define globally for SIRT reconstructions. This leads to noticeable variations of the diffusion coefficients in the case of SIRT reconstructions, whereas DART reconstructions resulted in more reliable data. In addition, the anisotropy of the determined diffusion properties was evaluated, which was significant in the presence of a limited missing wedge for SIRT and strongly reduced for DART

Coherent Electronic Wave Packet Motion in C-60 Controlled by the Waveform and Polarization of Few-Cycle Laser Fields

Citation: Li, H., Mignolet, B., Wachter, G., Skruszewicz, S., Zherebtsov, S., Sussmann, F., . . . Kling, M. F. (2015). Coherent Electronic Wave Packet Motion in C-60 Controlled by the Waveform and Polarization of Few-Cycle Laser Fields. Physical Review Letters, 114(12), 6. doi:10.1103/PhysRevLett.114.123004Strong laser fields can be used to trigger an ultrafast molecular response that involves electronic excitation and ionization dynamics. Here, we report on the experimental control of the spatial localization of the electronic excitation in the C-60 fullerene exerted by an intense few-cycle (4 fs) pulse at 720 nm. The control is achieved by tailoring the carrier-envelope phase and the polarization of the laser pulse. We find that the maxima and minima of the photoemission-asymmetry parameter along the laser-polarization axis are synchronized with the localization of the coherent electronic wave packet at around the time of ionization.Additional Authors: Tiggesbaumker, J.;Meiwes-Broer, K. H.;Lemell, C.;Burgdorfer, J.;Levine, R. D.;Remacle, F.;Kling, M. F

Magnetoelectric ordering of BiFeO3 from the perspective of crystal chemistry

In this paper we examine the role of crystal chemistry factors in creating conditions for formation of magnetoelectric ordering in BiFeO3. It is generally accepted that the main reason of the ferroelectric distortion in BiFeO3 is concerned with a stereochemical activity of the Bi lone pair. However, the lone pair is stereochemically active in the paraelectric orthorhombic beta-phase as well. We demonstrate that a crucial role in emerging of phase transitions of the metal-insulator, paraelectric-ferroelectric and magnetic disorder-order types belongs to the change of the degree of the lone pair stereochemical activity - its consecutive increase with the temperature decrease. Using the structural data, we calculated the sign and strength of magnetic couplings in BiFeO3 in the range from 945 C down to 25 C and found the couplings, which undergo the antiferromagnetic-ferromagnetic transition with the temperature decrease and give rise to the antiferromagnetic ordering and its delay in regard to temperature, as compared to the ferroelectric ordering. We discuss the reasons of emerging of the spatially modulated spin structure and its suppression by doping with La3+.Comment: 18 pages, 5 figures, 3 table

Advances in ab-initio theory of Multiferroics. Materials and mechanisms: modelling and understanding

Within the broad class of multiferroics (compounds showing a coexistence of magnetism and ferroelectricity), we focus on the subclass of "improper electronic ferroelectrics", i.e. correlated materials where electronic degrees of freedom (such as spin, charge or orbital) drive ferroelectricity. In particular, in spin-induced ferroelectrics, there is not only a {\em coexistence} of the two intriguing magnetic and dipolar orders; rather, there is such an intimate link that one drives the other, suggesting a giant magnetoelectric coupling. Via first-principles approaches based on density functional theory, we review the microscopic mechanisms at the basis of multiferroicity in several compounds, ranging from transition metal oxides to organic multiferroics (MFs) to organic-inorganic hybrids (i.e. metal-organic frameworks, MOFs)Comment: 22 pages, 9 figure

Effects of Vegetation, Corridor Width and Regional Land Use on Early Successional Birds on Powerline Corridors

Powerline rights-of-way (ROWs) often provide habitat for early successional bird species that have suffered long-term population declines in eastern North America. To determine how the abundance of shrubland birds varies with habitat within ROW corridors and with land use patterns surrounding corridors, we ran Poisson regression models on data from 93 plots on ROWs and compared regression coefficients. We also determined nest success rates on a 1-km stretch of ROW. Seven species of shrubland birds were common in powerline corridors. However, the nest success rates for prairie warbler (Dendroica discolor) and field sparrow (Spizella pusilla) were <21%, which is too low to compensate for estimated annual mortality. Some shrubland bird species were more abundant on narrower ROWs or at sites with lower vegetation or particular types of vegetation, indicating that vegetation management could be refined to favor species of high conservation priority. Also, several species were more abundant in ROWs traversing unfragmented forest than those near residential areas or farmland, indicating that corridors in heavily forested regions may provide better habitat for these species. In the area where we monitored nests, brood parasitism by brown-headed cowbirds (Molothrus ater) occurred more frequently close to a residential area. Although ROWs support dense populations of shrubland birds, those in more heavily developed landscapes may constitute sink habitat. ROWs in extensive forests may contribute more to sustaining populations of early successional birds, and thus may be the best targets for habitat management

Coherent Electronic Wave Packet Motion in C-60 Controlled by the Waveform and Polarization of Few-Cycle Laser Fields

Strong laser fields can be used to trigger an ultrafast molecular response that involves electronic excitation and ionization dynamics. Here, we report on the experimental control of the spatial localization of the electronic excitation in the C-60 fullerene exerted by an intense few-cycle (4 fs) pulse at 720 nm. The control is achieved by tailoring the carrier-envelope phase and the polarization of the laser pulse. We find that the maxima and minima of the photoemission-asymmetry parameter along the laser-polarization axis are synchronized with the localization of the coherent electronic wave packet at around the time of ionization.open113033sciescopu