Band 228, Heft 12

Polar molecular crystals seem to contradict a quantum mechanical statement, according to which no stationary state of a system features a permanent electrical polarization. By stationary we understand here an ensemble for which thermal averaging applies. In the language of statistical mechanics we have thus to ask for the thermal expectation value of the polarization in molecular crystals. Nucleation aggregates and growing crystal surfaces can provide a single degree of freedom for polar molecules required to average the polarization. By means of group theoretical reasoning and Monte Carlo simulations we show that such systems thermalize into a bi-polar state featuring zero bulk polarity. A two domain, i.e. bipolar state is obtained because boundaries are setting up opposing effective electrical fields. Described phenomena can be understood as a process of partial ergodicity-restoring. Experimentally, a bi-polar state of molecular crystals was demonstrated using phase sensitive second harmonic generation and scanning pyroelectric microscopy

Large crystals of Bi 2 M 4 O 9 (M = Fe, Ga) were examined by single-crystal neutron diffraction at the Institut Laue Langevin (ILL) in Grenoble. The crystal structures of these bismuth metal oxides are commonly described as mullite-type compounds in space group Pbam. However, up to several hundreds of reflections were observed in both compounds with significant intensity violating the reflection conditions in this space group. Psi-scans with a short wavelength (0.837 Å at D9) were quasi-continuous and thus provided ambiguous results, hampering the identification of the source of these intensities. With a much longer wavelength (2.364 Å at D10) the Psi scans displayed strong variations of the intensities with small changes of the scan angle Psi. These Psi scan intensities could be well matched by an appropriate calculation with the program UMWEG (Rossmanith, 2003) and thus multiple diffraction turned out to be the source of these extra reflections. As a result the space group Pbam proved to be the correct one. Due to widespread misconceptions the geometrical background of Umweganregung is discussed to considerable detail.

The crystal structure of keyite, ideally (Cu x ,Pb 1-x )Cu 2 Zn 4 Cd 2 (AsO 4 ) 6 · (H 2 O) 2x , has been refined in space group symmetries C2/c and C1̄. The keyite structure is compared with the alluaudite structure type, confirming that keyite belongs to the alluaudite group of minerals. While keyite is metrically monoclinic, ordering of Cu atoms and associated H 2 O within one of the channels of the alluaudite heteropolyhedral framework reduces the symmetry to triclinic and gives rise to weak X-ray diffraction maxima forbidden in space group C2/c. Streaks of diffuse scattering parallel b* may indicate additional ordering in layers normal to this direction. Pb 2+ partially replaces Cu 2+ within the channels, thus also decreasing the concentration of the water ligands coordinating Cu.

The bismuth 6s 2 lone electron pair (LEP) in mullite-type Bi 2 M 4 O 9/10 (where M = Al, Fe, or Mn) was characterized by means of several parameters derived from experimental and theoretical calculations. The Wang-Liebau eccentricity (WLE) parameter proved to be very useful to quantify the stereochemical activity of the LEPs. Calculations of electronic distributions (three-dimensional charge density difference isosurfaces) were used as independent measurements, which validated the relevance of the WLE parameter for the characterization and quantification of LEPs. The distribution of the Bi 6s 2 electrons around the nucleus was evaluated and the maximum of electron density calculated. The spatial orientation of this electron density with respect to the nucleus is expressed as “Liebau density vector”. Therefore, this vector is ascribed to be a key result of this work as a proof that the purely geometrically defined Wang-Liebau vector indeed points towards the maximum electron density of the LEP. The LEP stereochemical activity was studied in terms of the type of structure (Bi 2 M 4 O 9 or Bi 2 M 4 O 10 ) and the nature of M. The effect of exchanging bismuth by lanthanum as well as the relative stabilities of Bi 2 M 4 O 9 or Bi 2 M 4 O 10 structures were calculated and discussed.

The equiatomic intermetallic compounds AETMg and AETCd (AE = Ca, Sr; T = Pd, Ag, Pt, Au) and the whole solid solution Yb 2-x PtMg x were synthesized from the elements in sealed niobium tubes in a high-frequency or a muffle furnace. All samples were characterized on the basis of their powder X-ray diffraction patterns. The structures of SrAuCd, CaPdCd, CaPtCd, CaPdMg, SrPtMg, CaAg 1.017 Mg 0.983 , SrAg 1.032 Mg 0.968 , and Yb 1.792 PtMg 0.208 were refined on the basis of single-crystal X-ray diffractometer data. Some of the crystals showed small homogeneity ranges. All compounds crystallize with the orthorhombic TiNiSi type structure, space group Pnma. The crystal chemistry is briefly discussed and variations in chemical bonding as a result of the electronegativity of the transition metal are described.

The isotypic intermetallic compounds RE 2 RuMg 2 (RE = Dy, Ho, Er, Tm, Lu) were synthesized from the elements in sealed niobium tubes in a muffle furnace with different annealing sequences. The polycrystalline samples were characterized through Guinier powder patterns. The structure of Er 2 RuMg 2 was refined on the basis of single-crystal X-ray diffractometer data: ordered Os 2 Al 3 type, I4/mmm, a = 338.44(5), c = 2029.4(4) pm, wR2 = 0.0237, 234F 2 values, and 10 variables. The Er 2 RuMg 2 structure is an i5 superstructure of the bcc packing with complete ordering of the erbium, ruthenium, and magnesium atoms. Layers of compressed, ruthenium-centered erbium cubes (290 pm Er-Ru) alternate with layers of edge-sharing magnesium tetrahedra (311 and 338 pm Mg-Mg); a new type of magnesium substructure. Temperature dependent magnetic susceptibility studies confirm the stable trivalent ground states of the rare earth elements. Lu 2 RuMg 2 is a Pauli paramagnet and the remaining four compounds show Curie paramagnetism. Dy 2 RuMg 2 and Ho 2 RuMg 2 are accompanied by antiferromagnetic ordering at 8.9(1) and 3.6(1) K, respectively.

The metal-rich indide La 18 Co 28 In 3 was synthesized by melting of the elements in a high-frequency furnace and characterized by X-ray diffraction on powders and single crystals. This high-temperature phase crystallizes with a new structure type: I4/mmm, a = 1432.56(8), c = 984.35(6) pm, wR2 = 0.1054, 1027 F 2 values, 43 variables at 295 K. The structure is built up by a complex three-dimensional network of Co 4 tetrahedra which are condensed via further cobalt atoms with a broad range of Co-Co distances (242-262 pm). This network leaves larger channels which are filled by the lanthanum and indium centered polyhedra. The two crystallographically independent indium atoms have exclusively lanthanum neighbors within the channels. They form infinite (In1-In2-In2) chains along c with In1-In2 and In2-In2 distances of 332 and 319 pm, respectively. These chains are only weakly bonded to their lanthanum neighbors with La-In distances ranging from 344 to 368 pm. Both indium sites show enhanced displacements along the c axis. Temperature dependent diffraction studies and similarities to the incommensurate La 1.18 Rh 3 In 2 structure are discussed.

Crystal structures are reported for five racemic 4-{3-(halophenyl)hexahydro[1,3]oxazolo[3,4-a]pyridin-1- yl}-2,8-bis(trifluoromethyl)quinolines, 2, namely (2: X = 2-F, 3-Br, 4-Cl, 4-Br and 4-F), prepared from mefloquine and XC 6 H 5 CHO. In each case, the compound crystallizes in the triclinic space group, P1̄, with Z = 2. All molecules have ‘‘F’’ shapes with the X-phenyl and quinoline rings lying on one side of the best plane through the fused oxazolidine- piperidine rings. Differences in the conformations of the molecules are indicated by the differences in the interplanar angles. In each case, the supramolecular arrangements are made up from combinations of π ... π and some of C-H...X and C-X... π (X = halo) interactions. Two similar structural sub-sets found in the crystal structures of the five derivatives are centrosymmetric π ... π-linked dimers and centrosymmetric cage-like dimers. The cage-like dimers, formed by the intermeshing of the rings of the monomers, are generated from different weak intermolecular interactions, which include C-H...X and C-X... π (X = halo) interactions. Comparisons are made between the activities of the mefloquine- oxazolidine derivatives in in-vitro tests against the multidrug-resistant tuberculosis strain and conformations, as measured by interplanar angles.

A systematic series of compounds featuring bromo and nitro substituted derivatives or dimethoxy analogs of 4,4′-dihydroxybiphenyl are comparatively studied with reference to their crystal structures showing specific influence of the structural modification both on molecular conformation and packing of the molecules. While bromine substitution of the parent biphenol does not fundamentally affect the typical O-H...O generated layer structure, nitro substitution prevents corresponding layer formation due to intramolecular hydrogen bonding. On the other hand, in the case of the less strongly bound dimethoxy analogue, bromine substitution is strong enough to obstruct the C-H...O involved layer architecture, demonstrating a graded effect of substituents on the crystal structures of biphenol type compounds.