Sr2Si5N8 und Ba2Si5N8 werden phasenrein durch Umsetzung von Siliciumdiimid mit den jeweiligen Metallen Strontium bzw. Barium unter Stickstoffatmosphäre in einem speziellen Hochfrequenzofen bei 1 550 bis 1 650°C synthetisiert. Sr2Si5N8 (Pmn21, a = 571,0(2), b = 682,2(2), c = 934,1(2) pm, Z = 2, R = 0,037, wR = 0,021) und Ba2Si5N8 (Pmn21, a = 578,3(2), b = 695,9(2), c = 939,1(2) pm, Z = 2, R = 0,022, wR = 0,018) kristallisieren isotyp und sind aus M2+‐Ionen (M = Sr bzw. Ba) sowie einer kovalenten dreidimensionalen Raumnetzstruktur eckenverknüpfter SiN4‐Tetraeder aufgebaut. Je 4 N sind an zwei bzw. drei Si gebunden. Die an je zwei Si gebundenen N leisten den dominierenden koordinativen Beitrag für die M2+‐Ionen.
Using differential scanning calorimetry (DSC) measurements in combination with structural and optical characterization we have investigated the formation conditions of different phases of tris(8‐hydroxyquinoline)aluminum (Alq3). We have identified the δ‐phase as a high‐temperature phase of Alq3 being composed of the facial stereoisomer, and report an efficient method to obtain blue luminescent Alq3 by a simple annealing process. This allows the preparation of large amounts of pure δ‐Alq3 by choosing appropriate annealing conditions, which is necessary for further characterization of this blue‐luminescent phase, and offers the possibility of fabricating blue organic light‐emitting devices (OLEDs) from this material.
We report on two different crystalline phases of tris͑8-hydroxyquinoline͒ aluminum (Alq 3) which were obtained by thermal sublimation in a horizontal glass tube. These phases are investigated by x-ray powder diffraction, Raman and infrared spectroscopy, and low temperature photoluminescence measurements. Apart from the already known ␣ phase we could identify a new crystalline phase of Alq 3 (␦-Alq 3) showing blueshifted fluorescence. As compared to the ␣ phase this new phase is characterized by a larger unit cell volume, a reduced number of Raman lines in the energy range between 70 and 700 cm Ϫ1 , a blueshift of the photoluminescence maximum by about 0.2 eV, and a decreased intersystem crossing to the triplet state. These differences are interpreted in terms of the isomery of the Alq 3 molecule. It is supposed that the new phase contains the facial isomer, whereas in the other phases only the meridianal isomer was reported. Low temperature photoluminescence spectra show a well-resolved vibronic progression with about the same spacing of 550 cm Ϫ1 for both crystalline phases of Alq 3. Site-selective photoluminescence measurements reveal the existence of an additional redshifted featureless emission, which is ascribed to energy relaxation into low-lying states.
Crystal structures of both one-and two-layer hydrates of sodium fluorohectorite were refined against single crystal data for the first time because melt synthesis yielded a sodium fluorohectorite showing little stacking disorder as compared to natural clays. In both hydrate phases, the relative shift of adjacent 2:1 layers is fixed by hydrogen-bonding between water molecules coordinated to interlayer cations and basal oxygen atoms of tetrahedral sheets encompassing the interlayer space. Despite some apparent diffuse scattering, a decent single crystal refinement of the semi-ordered structure of the onelayer hydrate is achieved, revealing structural details of the interlayer spacing for the first time. For the two-layer hydrate the structural model proposed for vermiculites is confirmed but a different ordering pattern of interlayer [Na(H 2 O) 6 ] + is suggested. While in the two-layer hydrate sodium cations reside at the centre of the interlayer space, in the one-layer hydrate sodium is displaced from the centre of the interlayer space either towards the upper or towards the lower tetrahedral sheet. This displacement allows for coordination to the hexagonal cavity on one side while the coordination sphere of sodium is completed by three coordinating water molecules on the other side. These three water molecules in turn are involved in hydrogen bonding to the opposite tetrahedral sheet.
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