A. Kirfel

Electron-density distribution in stishovite, SiO2: a new high-energy synchrotron-radiation study

The electron-density distribution of the high-pressure polymorph of SiO2, stishovite [a = 4.177 (1), c = 2.6655 (5) A, space group P4(2)/mnm, Z = 2], has been redetermined by single-crystal diffractometry using synchrotron radiation of 100.42 and 30.99 keV, respectively, in order to obtain essentially absorption- and extinction-free data. Room-temperature diffraction experiments on two samples of irregular shape were carried out on two different diffractometers installed at HASYLAB/DESY, Hamburg, Germany. The structure refinement on the high-energy data converged at R(F) = 0.0047, wR(F) = 0.0038, GoF = 0.78, for a multipole model with neutral atoms and multipole expansions up to seventh order. For each atom, the radial expansion coefficients of the multipole orders (l > 0) were constrained to a common value. The absence of extinction was indicated by a refined correction parameter equalling zero within error limit. The excellent quality of the data is also illustrated by a high-order (HO) refinement (s > 0.7 A(-1)) yielding R(F) = 0.0060, wR(F) = 0.0048, GoF = 0.85. Both static deformation electron-density distribution and structure amplitudes compare well with corresponding results obtained from band-structure calculations using the linearized-augmented-plane-wave (LAPW) method. Ensuing topological analysis of the total model electron density distribution revealed bond critical point properties for the two unique Si--O bonds, indicating a predominantly closed-shell interaction mixed with a significant shared interaction contribution that decreases with increasing interatomic distance. Calculation of atomic basins yielded charges of +3.39 e and -1.69 e for Si and O, respectively, in good agreement with the theoretically calculated values of +3.30 e and -1.65 e. The volumina of the Si and O basins are 2.32 and 10.48 A3, corresponding to spheres with radii of 0.82 and 1.36 A, respectively. The results also conform well with correlations between bond length and bond critical point properties reported in the literature for geometry-optimized hydroxyacid molecules. Estimates of the Si cation electronegativity indicate that the change of Si coordination by oxygen from 4 to 6 is accompanied by an increase of the ionicity of the Si--O bond of about 7%.

Low-temperature anomalies of cuprite observed by Raman spectroscopy and x-ray powder diffraction

Anomalous low-temperature behaviour of Cu2O is observed by both Raman-spectroscopy using laser excitation frequencies near the resonance of the 1S ‘blue exciton’ and x-ray powder diffractometry using a high-resolution Guinier film camera. Changes in frequency of the Raman-allowed Γ25+(LO) phonon band, and also of intensities of other Raman-forbidden bands, indicate anomalies at temperatures of 90 and 190 K, respectively. The anomaly at 90 K is qualitatively interpreted by an order–disorder phase transition and that at 190 K by a discontinuous change of excitonic states. Close to these temperatures, the powder diffraction diagrams also revealed anomalous behaviour of the lattice constant. Upon cooling and heating, the thermal expansion coefficient becomes significantly negative at 200 K, and at about 80 K a small reproducible contraction/expansion of the cubic lattice is observed. Both effects add evidence for hitherto unknown instabilities of the Cu2O structure at low temperatures.

Structure analyses of Ba-silicate glasses

Abstract The structures of four different Ba-silicate glasses were analysed applying a broad spectrum of diffraction and spectroscopic methods, atomic force microscopy and computer simulations. This collaborative study offers new insights into the influence of the network modifier BaO. 3 at.% carbon were incorporated into two of the glasses with the intention to obtain additional information about the effect of partial substitution of carbon for oxygen on the considered structures. A structure model for Ba-silicate glasses with compositions close to BaSi 2 O 5 is discussed that differs with respect to other models proposed in previous studies. We propose that [SiO 4 ] tetrahedra do not exclusively form six-membered rings, but also considerable numbers of smaller rings arranged in (slightly) folded layers which alternate with barium layers and/or barium chains. Additionally, the occurrence of tetrahedral chains, possibly isolated rings and of regions in which the [SiO 4 ] tetrahedra form a silica-like network is assumed.

Synthesis of trans-epoxy-l-proline and cis-aziridino-l-proline from S -pyroglutamic acid. Regio- and diastereoselective ring opening of its derivatives

Abstract The pyroglutamic acid derivative 4 was converted through several steps into 2 S ,3 R ,4 S -epoxyproline 8 . Key steps of the reaction sequence were the stereoselective epoxidation of 4 to 5 and the chemoselective reduction of the amide group of 5 with concomitant transformation of the acetal moiety into the N-benzyl protecting group without oxirane ring opening. The air sensitive benzyl derivative was transformed to the stable N-Boc prolinol derivative 6 . Oxidation of 6 gave the protected epoxyproline derivative 7 . Deprotection of 7 furnished enantiopure 2 S ,3 R ,4 S -epoxyproline 8 . Ring opening of the oxirane 6 or 16 was accomplished with C, N, Cl-nucleophiles under complete regiocontrol. Azidoprolinol 9 served as a starting material for the synthesis of epiminoproline derivative 23 .

Quaternary Tröger bases as new inclusion hosts; the first X-ray structures of a Tröger base and of a dioxane clathrate

New clathrates of various onium salts (2)–(5), (7), and (8)of the Troger bases (1) and (6) are described, including X-ray structures of the free host (6), and of an inclusion compound of (2b) with dioxane.

Experimental Bond Critical Point and Local Energy Density Properties Determined for Mn-O, Fe-O, and Co-O Bonded Interactions for Tephroite, Mn2SiO4, Fayalite, Fe2SiO4, and Co2SiO4 Olivine and Selected Organic Metal Complexes: Comparison with Properties Calculated for Non-Transition and Transition Metal M-O Bonded Interactions for Silicates and Oxides

Bond critical point (bcp) and local energy density properties for the electron density (ED) distributions, calculated with first-principle quantum mechanical methods for divalent transition metal Mn-, Co-, and Fe-containing silicates and oxides are compared with experimental model ED properties for tephroite, Mn 2SiO 4, fayalite, Fe 2SiO 4, and Co 2SiO 4 olivine, each determined with high-energy synchrotron single-crystal X-ray diffraction data. Trends between the experimental bond lengths, R(M-O), (M = Mn, Fe, Co), and the calculated bcp properties are comparable with those observed for non-transition M-O bonded interactions. The bcp properties, local total energy density, H( r c), and bond length trends determined for the Mn-O, Co-O, and Fe-O interactions are also comparable. A comparison is also made with model experimental bcp properties determined for several Mn-O, Fe-O, and Co-O bonded interactions for selected organometallic complexes and several oxides. Despite the complexities of the structures of the organometallic complexes, the agreement between the calculated and model experimental bcp properties is fair to good in several cases. The G( r c)/rho( r c) versus R(M-O) trends established for non-transition metal M-O bonded interactions hold for the transition metal M-O bonded interactions with G( r c)/rho( r c) increasing in value as H( r c) becomes progressively more negative in value, indicating an increasing shared character of the interaction as G( r c)/rho( r c) increases in value. As observed for the non-transition metal M-O bonded interactions, the Laplacian, nabla (2)rho( r c), increases in value as rho( r c) increases and as H( r c) decreases and becomes progressive more negative in value. The Mn-O, Fe-O, and Co-O bonded interactions are indicated to be of intermediate character with a substantial component of closed-shell character compared with Fe-S and Ni-S bonded interactions, which show greater shared character based on the | V( r c)|/ G( r c) bond character indicator. The atomic charges conferred on the transition metal atoms for the three olivines decrease with increasing atomic number from Mn to Fe to Co as the average M-O bond lengths decrease from 2.219 to 2.168 to 2.128 A, respectively.

The Jülich neutron diffractometer and data processing in rock texture investigations

Abstract The texture diffractometer SV7-b which is operated at the Forschungszentrum Julich as a user and service instrument has been improved with respect to the efficiency of the position-sensitive detector system and to the variety of wavelengths available. Instrumental aspects of data collection and different methods of data processing for pole figure evaluation are discussed. Recent examples of texture investigations on geological materials are presented for quartz, hematite, hematite-quartz, orthopyroxen-quartz and anorthosite rocks.

Neutron and synchrotron radiation studies of archaeological objects

This chapter focuses on the neutron and synchrotron radiation studies of archaeological objects. Time-of-flight neutron and synchrotron X-ray diffraction are used for the fingerprint determinations and quantitative mineral phase analysis of archaeological objects. Both neutron and X-ray diffraction techniques have their advantages and drawbacks when used in archaeological research. Neutron diffraction allows non-destructive analysis of complete and unprepared objects. Synchrotron X-ray diffraction can be used for fast and high-resolution data collection on small amounts of powder samples, surfaces, or thin sections. This chapter concentrates on the introduction of the white-beam neutron diffraction technique applied on archaeological pottery. X-ray diffraction results from both laboratory and synchrotron sources are given for comparison. X-ray and neutron diffraction are well known experimental methods for investigating minerals, or other inorganic and organic materials wherever their crystal structures are the matter of interest in the wide field of material science and industrial applications. The knowledge of the crystal structure of a material or the abundance of different known phases in a multi-phase mixture of minerals are not only fundamental for the understanding of the physical and chemical properties of the material, but can also help to fingerprint the origin of objects of archaeological interest. It concludes that neutron and SR X-ray radiation is capable of providing high quality diffraction fingerprints of archaeological ceramics.

The twin-diffractometer SV7 at the FRJ-2 as a workhorse for structure and texture research

Abstract Instrumental set-up, physical characteristics and applications of the refurbished twin-diffractometer SV7, a powder and a texture diffractometer, at the FRJ-2 in Julich are summarized. New or renewed components concern monochromators, neutron beam path, sample environment, detectors, and electronic control. Use and beam time organisation of the diffractometers as national service instruments are discussed.

CATION DISTRIBUTIONS IN OLIVINES AND ORTHOPYROXENES. AN INTERLABORATORY STUDY

The determination of the (Fe, Mg)-cation distribution in important rock forming minerals such as olivines and orthopyroxenes from single crystal X-ray diffraction data is well established. The findings obtained from good quality data are, however, subject to the structure model, refinement strategy, data weighting etc. Moreover, they do not necessarily agree with those of comparable Mössbauer studies. In order to assess the reliability of the X-ray results, i.e. precision and accuracy of the site occupancies and ensuing cation distribution coefficients KD, an interlaboratory study was undertaken. Two olivine and two orthopyroxene data sets, one experimental and three synthetic, were dispatched to interested applicants for structure refinements according to their own choice. Approximately correct structure parameters were given, but no information about the chemical composition, i.e. the Fe-content, was communicated. The total of 25 refinements for each data set, carried out with different programs, valence models, and weighting schemes, yielded some considerable differences between the results, which are summarized and discussed in terms of precision and accuracy. The uncertainties of the refined site occupancies, following from the qualities of the fits, are only realistic, when they are dominated by the statistical errors in the data. Errors in the independently refined Fe-concentrations on the M1-and M2-sites are not fully transferred to the thermodynamically relevant cation distribution coefficient. Therefore, correlations between the site occupancies, clearly recognizable from the compilations of all respective results, can be combined with an independently obtained chemical constraint (microprobe analysis) in order to yield reliable KD-values with realistic uncertainties. In summary, all results indicate the need for multiple refinement techniques, if reliable results on the absolute scale are desired.