Band 149, Heft 1-2

The quadratic electrooptic and electrostrictive constants of NH 3 CH 3 Al(SO 4 ) 2 · 12H 2 O (MASD) have been measured between room temperature and the ferroelectric phase transition at −96° C. Both effects increase strongly with decreasing temperature. A thermodynamic analysis of the results reveals the dominant influence of electrostriction on all electrooptic properties. The thermal behaviour of the effects is not compatible with a simple order parameter like the polarization P . Therefore we suggest an intrinsic order parameter θ which couples strongly with P . This assumption allows a correct description of the temperature dependence of the electrostrictive constants above the transition temperature. Thus MASD must be classified as an improper ferroelectric crystal. The essential features of the quadratic electrooptic effect found with MASD also occur in other methylammonium alums.

The thermal expansion of SnS and SnSe has been studied above room temperature up to the melting point of 1163 ± 5K and 1135 ± 5K, respectively, by X-ray diffraction techniques using a 190 mm Unicam high temperature camera. The changes of the lattice parameters indicate that the atomic positions in the (010) plane approach a square planar arrangement with increasing temperature. The transformation of SnS and SnSe from orthorhombic to a pseudotetragonal orthorhombic modification with a ≈ c < b is observed at 878 ± 5K and 807 ± 5K, respectively. The lattice parameters of the high temperature modifications are for SnS [unk] = 4.162 ± 0.006Å, b = 11.517 ± 0.015Å at 932 K and for SnSe [unk] = 4.313 ± 0.006Å, b = 11.703 ± 0.015Å at 820 K. Both compounds maintain this structure type up to the melting points.

γ Ag 8 GeTe 6 belongs to space group F [unk]3 m with Z = 4 and a = 11.58 ± 0.02Å. The arrangement of the tellurium atoms is almost precisely that of both the magnesium and copper atoms in the Friauf-Laves cubic phase of MgCu 2 . This arrangement produces 136 tetrahedra per unit cell, of which four are occupied by the Ge atoms. The remainder are divided among two 48-fold, two 16-fold and one 4-fold sets of positions. One of the 16-fold sets of tetrahedra which shares faces with the GeTe 4 tetrahedra is empty. The Ag atoms are distributed over the remaining sets with 29 of the 32 in the unit cell being in the two 48-fold sets and 25 in just one of these. There are many routes whereby the Ag atoms may move through the crystal; in particular the two 48-fold sets themselves form such routes. Not all the Ag atoms are constrained to lie at the near-centers of the tetrahedra; some are closer to particular shared faces. There are substantial differences between the structure reported here and that reported earlier by Rysanek, Laruelle and Katty.

The structure of the title compound, C 15 H 14 NOBr was elucidated using X-ray diffraction technique. The crystals are orthorhombic, space group P 2 1 2 l 2 l with a = 7.73(1), b = 29.83(2), c = 5.92(1)Å, Z = 4. The structure was solved by the heavy-atom method followed by a block-diagonal and full-matrix least-squares refinement to an R of 0.043 for 983 counter reflexions. The S configuration of α -phenylethylamine asymmetric center allowed us to establish the absolute configuration of the examined compound as (+)−2/ R , 3 R −2( S -l-phenylethyl)-3- p -bromophenyloxaziridine.

The structure of lactarorufin B is verified by X-ray analysis. Crystals of the title compound, C 22 H 23 O 5 Br, are monoclinic, a = 16.55, b = 6.75, c = 20.07 Å, β = 111.7°, Z = 4, space group C2. The intensities were collected on a diffractometer using Cu Kα radiation. A Patterson synthesis revealed the heavy-atom position. A block-diagonal and full-matrix least-squares refinement with anisotropic temperature factors for non-hydrogen atoms led to a final R of 6.9 %. The results indicate that the lactone carbonyl oxygen occurs at C(5) and not at C(13) as was deduced earlier from chemical and spectroscopic evidences.

The problem of the detection of hydrido-hydrogen atoms in transition metal complexes using F [unk] and Δ F Fourier maps is tackled. To date, the F [unk] Fourier maps have been neglected for the solution of this problem, but in conjunction with the Δ F syntheses, these maps are found to be very useful for locating hydrido hydrogen atoms. For centrosymmetric space groups, in cases where parts of the structure, even at the end of refinement, are poorly determined, hydride hydrogen atoms are located better by the F [unk] Fourier than by the Δ F Fourier.