G. Chapuis

SUPERFLIP – a computer program for the solution of crystal structures by charge flipping in arbitrary dimensions

SUPERFLIP is a computer program that can solve crystal structures from diffraction data using the recently developed charge-flipping algorithm. It can solve periodic structures, incommensurately modulated structures and quasicrystals from X-ray and neutron diffraction data. Structure solution from powder diffraction data is supported by combining the charge-flipping algorithm with a histogram-matching procedure. SUPERFLIP is written in Fortran90 and is distributed as a source code and as precompiled binaries. It has been successfully compiled and tested on a variety of operating systems.

Dynamics of the n‐decylammonium chains in the perovskite‐type layer structure compound (C10H21NH3)2CdCl4

The two structural phase transitions in the perovskite layer structure compound (C10H21NH3)2CdCl4 at Tc1=35 °C and Tc2=39 °C have been studied by x‐ray diffractional, calorimetric and dielectric measurements, proton NMR spin–lattice relaxation and second moment investigations, 35Cl and 14N quadrupole resonance spectroscopy as well as group theoretical considerations. The results show that the transition (Tc1) from the low temperature phase (P21/n) to the intermediate temperature phase (Pmnn) is basically a dynamic order–disorder transition of rigid alkyl chains, whereas the transition (Tc2) from the intermediate to the high temperature phase (Amaa) is connected with a melting of the alkyl chains. In the intermediate phase the rigid alkyl chains are flipping around their long axes between two equivalent orientations separated by 90°. The two transitions are somewhat analogous to the ones found in the lipid bilayer membranes and can be described by order parameters used for smectic liquid crystals.

2‐cyclooctylamino‐5‐nitropyridine, a new nonlinear optical crystal with orthorhombic symmetry

A new nonlinear optical crystal 2‐cyclooctylamino‐5‐nitropyridine (COANP) with point group symmetry mm2 has been grown by a temperature difference solution growth technique. Its linear and nonlinear optical properties have been investigated. The refractive indices na =1.702, nb =1.847, and nc =1.681 (λ=550 nm) and the nonlinear optical susceptibilities d31 and d33 (λω =1.06 μm) have been determined. Type I phase matching for Nd:YAG lasers (λ=1.06 μm) has been demonstrated for the configuration using a combination of d31 and d32 by angle tuning. A peak efficiency η=P2ω/Pω =3.6% has been observed with a 0.90‐mm‐thick crystal and a fundamental power Pω =560 W.

Phase Transitions in Disordered Lead Iron Niobate - X-Ray and Synchrotron Radiation Diffraction Experiments

Diffraction above room temperature, on powders and crystals of Pb(Fe0.5Nb0.5)O-3, using X-rays and synchrotron radiation, led to a novel and consistent picture of the symmetry of the phases in this disordered perovskite. The compound is a monoclinic ferroelectric at 293 K, undergoes a structural transition to another ferroelectric, tetragonal state at about 355 K and finally transforms into a cubic form at approximately 376 K

Linear- and nonlinear-optical properties of 2-cyclooctylamino-5-nitropyridine

The linear- and nonlinear-optical properties of 2-cyclooctylamino-5-nitropyridine (COANP) have been investigated. The refractive indices exhibit strong dispersion and a high birefringence (0.15–0.3) in the visible. The three main refractive indices at λ = 550 nm are na = 1.700, nb = 1.845, and nc = 1.683. The calculations that were carried out show interesting critical (frequency doubling of the Nd:YAG laser at λ = 1064 nm) and noncritical [sum-frequency generation using 488-nm (argon) and 647-nm (krypton)] phase-matching properties. The nonlinear-optical susceptibilities d32 = (32 ± 16) pm/V and d33 = (13.7 ± 2) pm/V were determined. A one-dimensional charge-transfer model can be used to explain the anisotropy of the nonlinear-optical susceptibility tensor of COANP qualitatively.

The modulated structure of Ba0.39Sr0.61Nb2O6. I. Harmonic solution

The structure of a crystal of Sr(0.61)Ba(0.39)Nb(2)O(6) has been solved and refined as an incommensurate structure in five-dimensional superspace. The structure is tetragonal, superspace group P4bm(pp1/2,p - p1/2), unit-cell parameters a = 12.4566 (9), c = 7.8698 (6) A, modulation vectors q(1) = 0.3075 (6) (a* + b*), q(2) = 0.3075 (6) (a* - b*). The data collection was performed on a KUMA-CCD diffractometer and allowed the integration of weak first-order satellite reflections. The structure was refined from 2569 reflections to a final value of R = 0.0479. The modulation affects mainly the positions of the O atoms, which are displaced by as much as 0.5 A, and the site 4c that is occupied by Sr and Ba atoms. Only a simplified model, in which this atomic position is occupied by an effective atom Sr/Ba, could be refined from the data set. The modulation of displacement parameters has been used to account for the modulated distribution of Sr and Ba. The whole refinement uses only first-order modulation waves, but there are strong indications that for a complete solution the use of higher-order satellites and a more complicated model is necessary.

Optical Second-Harmonic Generation in Single Crystals of Thiosemicarbazide Cadmium Bromide Hydrate (Cd(NH2NHCSNH2)Br2?H2O)

A new nonlinear optical organic-inorganic crystal, thiosemicarbazide cadmium bromide hydrate (TSCCB), with crystal class m has been grown by a temperature-lowering technique (TLT) and a temperature-difference technique (TDT). The crystal structure of TSCCB is reported. Its linear and nonlinear optical properties have been investigated. The highest measured nonlinear optical susceptibility is dmax = 2.8 pm/V.

Five-dimensional structure refinement of natural melilite, (Ca1.89Sr0.01Na0.08K0.02)(Mg0.92Al0.08)(Si1.98Al0.02)O7

The structure of a crystal of natural melilite from San Venanzo, Umbria (Italy) of the general formula X2T1(T2)2O7, where X = Ca0.945Sr0.005Na0.04K0.01, T1 = Mg0.92Al0.08 and T2 = Si0.99Al0.01, has been solved and refined as an incommensurate structure in five-dimensional superspace. The structure is tetragonal, superspace group P\bar 421m:p4mg, cell parameters a = 7.860 (1), c = 5.024 (1) A, modulation vectors q1 = 0.2815 (3)(a* + b*), q2 = 0.2815 (3)(−a* + b*). The data collection was performed on a KumaCCD diffractometer. The structure was refined from 7606 reflections to final R = 0.0481. A special modification of the refinement program Jana2000 was necessary to take into account overlapping of satellite reflections m × n = ±1, which could not be properly separated in the integration procedure. The final model includes modulations of the atomic positions as well as modulations of the thermal parameters. The latter are induced by strong differences in the neighbourhood of the actual modulated positions. The occupational modulation was neither significant for X nor for T1 sites and the sites were supposed to be occupied only by Ca and Mg, respectively. As a consequence of the Ca and O positional modulations six-, seven- and eightfold Ca coordination occur throughout the structure and the thermal ellipsoid changes its shape correspondingly. The positional modulation of the atoms causes variations in the interatomic distances which, however, do not affect bond-valence sums considerably, but induce flattening and rotation in T1 and T2 tetrahedra, respectively.

The luminescence of NaxEu3+(2−x)/3MoO4 scheelites depends on the number of Eu-clusters occurring in their incommensurately modulated structure

Scheelite related compounds with general formula Mn(XO4)m are the subject of hefty interest owing to their optical properties, stability and relatively simple preparation. Eu3+-containing scheelites are considered as red-emitting phosphors and the main factors affecting their luminescence are thought to be chemical composition and particle size while the influence of their structure is generally ignored. Here we report eight compounds from the NaxEu(2−x)/3MoO4 series prepared by conventional solid-state reaction and present a detailed analysis of their crystal structures. Six of them have modulated structures, a common feature of SRCs, in which dopant Eu3+ ions are orderly distributed. Moreover, different amounts of Eu3+ dimers are detected in the modulated structures, characterized by weak satellite reflections appearing in the lower angle part of the XRD patterns. These reflections are indexed and incorporated into Rietvelds refinement using superspace (3 + 1)-dimension symmetry. The remarkable feature of the compounds is that the characteristic luminescence parameters, overall (QEuL) and intrinsic (QEuEu) quantum yields, Eu(5D0) lifetimes, and sensitization efficiencies (ηsens), correlate with the number of Eu3+ aggregates, but not directly with the composition x of the materials. This provides an efficient tool for understanding and controlling the luminescence properties of scheelite related compounds.

Iron and ruthenium carbonyls of 2,3,5,6-tetrakis(methylene)bicyclo[2.2.2]octane. Crystal and molecular structure of (C12H14)Fe(CO)3

Abstract The reaction of 2,3,5,6-tetrakis(methylene)bicyclo[2.2.2]octane (1a) with Fe 2 (CO) 9 yields the two (η 4 -1,3-diene)Fe(CO) 3 isomers (2: exo; 3:endo) together with two bimetallic isomers (C 12 H 14 )[Fe(CO) 3 ] 2 (4:endo-exo; 5:diexo). The reaction of 1a with Ru 3 (CO) 12 yields the endo-(C 12 H 14 )Ru(CO) 3 (6) and endo, exo-(C 12 H 14 [Ru(CO) 3 ] 2 (7) complexes as the main products. The molecular structure of 3 has been determined by X-ray crystallography. The Fe(CO) 3 group is in the endo position with respect to the roof-shaped tetraene. The ligand is bound through one diene group to two basal positions of a tetragonal pyramidal Fe(CO) 3 L 2 moiety. Hydrogen atom positions have been determined (final residual R = 0.029). The dimensions for the 1,3-butadieneirontricarbonyl system, as found in this and 41 other structures, are summarized and discussed statistically. In this study the weighted averages for all structures show the threeCC distances to be of equal length and the FeC (inner) distance to be shorter than the FeC (outer) distance. The deviations of H(Z) and H(E) atoms from the butadiene plane, as found in this and 6 other structures having an exocyclic unsubstituted 1,3-diene group, are also discussed. Neither thermal epimerization of iron nor epimerization catalyzed by H + were found for complexes 2 – 7 whose structures in solution were deduced from their 1 H and 13 C NMR data on the basis of the known structure of 3.