Using x-ray and neutron diffraction data, the degree of order of the octahedral site cations has been determined for the perovskites Sr2AlNbO6 and Sr2AlTaO6, which have been prepared by several different methods and annealed at temperatures up to 1690 °C. The degree of order generally increases with increasing synthesis temperature. The amount of cation ordering is, therefore, primarily controlled by kinetic processes and not by thermodynamic equilibrium considerations. Increased order obtained with increased heating time confirms this general kinetic limitation on the degree of order. However, annealing Sr2AlNbO6 in the highest temperature region resulted in some decrease in order, presumably due to thermodynamic considerations. The cubic edge of both compounds decreases significantly with increasing order. Ordered domains are separated by antiphase boundaries which occur in high concentrations. The cubic cell edge within the ordered domains is significantly smaller than the overall cell edge when the concentration of antiphase boundaries is high. The antiphase boundaries cause significant lattice strain which generally decreases as the concentration of antiphase boundaries decreases. Results on other A2M3+M5+O6 systems are briefly presented.

Highly strained, optically active (E)-cycloheptene (1E) was prepared for the first time in the enantiodifferentiating geometrical photoisomerization of the (Z)-isomer (1Z) sensitized by chiral benzenetetracarboxylates at −40 to −80 °C. Low-temperature irradiations of 1Z in the presence of the chiral sensitizer, and subsequent stereospecific trapping of the optically active photoproduct, 1E, through a Diels−Alder reaction with 1,3-diphenylisobenzofuran or by oxidation with OsO4, afforded the cycloadduct or trans-1,2-cycloheptanediol, respectively. The enantiomeric excesses (ee's) of the two products were subsequently determined by chiral HPLC or GC. The ee of the product, which was used as a measure of the efficiency of chirality transfer in the excited state, was found to depend critically not only on the chiral sensitizer employed but also on the temperature and solvent employed. Thus, the ee of the product was doubled in an extreme case simply by changing the solvent from dichloromethane to hexane. Furthermore, the product chirality could be switched over a relatively narrow range of temperature as a consequence of the significant contribution of the entropy term in the enantiodifferentiating isomerization within the exciplex intermediate. Sensitization with (−)-bornyl benzenetetracarboxylate in hexane at −80 °C gave an ee value of 77%, which is the highest ee ever obtained for an asymmetric photosensitization. Based on the differential activation enthalpy and entropy for the enantiodifferentiating process and the fluorescence quenching experiments with C5−C8 cycloalkenes, the origin of the highly efficient enantiodifferentiation and a detailed mechanism for the enantiodifferentiating photoisomerizations are discussed.

The germanide Eu(2)AuGe(3) was obtained as large single crystals in high yield from a reaction of the elements in liquid indium. At room temperature Eu(2)AuGe(3) crystallizes with the Ca(2)AgSi(3) type, space group Fmmm, an ordered variant of the AlB(2) type: a = 857.7(4), b = 1485.5(10), c = 900.2(4) pm. The gold and germanium atoms build up slightly distorted graphite-like layers which consist of Ge(6) and Au(2)Ge(4) hexagons, leading to two different hexagonal-prismatic coordination environments for the europium atoms. Magnetic susceptibility data showed Curie-Weiss law behavior above 50 K and antiferromagnetic ordering at 11 K. The experimentally measured magnetic moment indicates divalent europium. The compound exhibits a distinct magnetic anisotropy based on single crystal measurements and at 5 K it shows a metamagnetic transition at ∼10 kOe. Electrical conductivity measurements show metallic behavior. The structural transition at 130 K observed in the single crystal data was very well supported by the conductivity measurements. (151)Eu Mössbauer spectroscopic data show an isomer shift of -11.24 mm/s at 77 K, supporting the divalent character of europium. In the magnetically ordered regime one observes superposition of two signals with hyperfine fields of 26.0 (89%) and 3.5 (11%) T, respectively, indicating differently ordered domains.

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