Volume 215, Issue 9

This paper deals with transmission electron microscope experiments of Ca,Sr-åkermanite solid solutions at temperatures between 100 K and 375 K. The aim of the investigations was to study the compositional and temperature dependence of phase transitions from the normal to the incommensurately modulated structure of (Ca 1-x Sr x ) 2 MgSi 2 O 7 single crystals observed by electron diffraction. With increasing Sr-content the normal to incommensurate phase transition temperature decreases linearly from ca. 350 K (x = 0.04) to 295 K (x = 0.22). Depending on temperature and composition, the incommensurate phase is structurally not uniform, its stability field can be divided into two regions. The transition between the two regions can be recognized by the additional appearance of higher order satellite reflections in the diffraction pattern. Additionally, the transition is characterized by an abrupt change of the modulation wavelength. Within the high-temperature, low ordered region, the modulation wavelength varies between 18.5 Å and 20.5 Å, whereas no changes of the wavelength have been found in the low-temperature, higher ordering region. With increasing Sr-content the high-temperature, low ordered phase covers a steadily increasing temperature range.

The introduction of area detectors has led to a significant reduction of the time needed for measuring a complete set of X-ray reflections. The default parameters suggested by the manufacturer of the instrument lead to good results, but both the software and the hardware provide the possibility of varying numerous parameters in order to optimize the quality of the data set. The influence of parameters (measuring time, scan angle, crystal-to-detector distance, collimator radius, detector resolution, redundancy, crystal size and scan type) is determined and rated with regard to their importance for the quality of the data. The investigations were carried out on a Siemens (now Bruker AXS) CCD diffractometer on three small organic molecules, p-nitrobenzaldehyde diacetate [P2 1 /n; Z=4; a=10.785(2) Å; b=7.420(2) Å; c=15.407(2) Å;β=107.30(1)°] and p-methoxyacetophenone [P2 1 /c; Z=4; a=10.158(2) Å; b=7.917(2) Å; c=10.287(2) Å; β=103.72(3)°], of which the crystal structures were determined. An optimized strategy for the data collection is proposed and the attainable improvement is shown by comparing the data sets based on the default parameters and the data sets obtained with the optimized strategy for the three structures examined. However, different strategies may be needed for more strongly absorbing materials.

The crystal structures of the new gallosilicate hydrosodalites Na 5 [Ga 0.8 Si 1.2O4 ] 6 (H 2 O) 8 was refined from X-ray powder data in the bodycentred space group I-43m: a = 884.15 pm, V = 0.6912 nm 3 , Z = 2, R WP = 0.063, R P = 0.048, R I = 0.014 for 6000 data points and 12 variable positional parameters. The statistical distribution of the gallium and silicon atoms on the tetrahedral framework positions was proved by 29 Si MAS NMR. TG/DSC experiments and in situ X-ray heating measurements indicate a phase transition of this compound to the phase Na 0.8 Ga 0.8 Si 1.2O4 . The sodalite Na 6 [GaSiO 4 ] 6 (H 2 O) 8 was reinvestigated from X-ray powder data in the space group P-43n: a = 885.92 pm, V = 0.6953 nm 3 , Z = 2, R WP = 0.047, R P = 0.033, R I = 0.017 for 2000 data points and 10 variables. Here the tetrahedral atoms are completely ordered as shown in the 29 Si MAS NMR spectrum. Additionally, the structure was refined from single crystal diffractometer data using a very small crystal. This refinement resulted in: a = 887.8 pm, V = 0.6998 nm 3 , Z = 2, wR 2 = 0.046, R = 0.043 for 294 observed data and 24 variable parameters. Heating this compound to 1173 K shows a phase transition which resulted in this case in the phase NaGaSiO 4 -I. This phase was also refined from powder data in the hexagonal space group P6 3 : a = 872.9 pm, c = 820.8 pm V = 0.5416 nm 3 , Z = 6, R WP = 0.069, R P = 0.047, R I = 0.026 for 2000 data points and 20 variables. In comparison to similar structures reported before, the hexagonal space group was chosen. Because two reflections could not be explained with this symmetry, it is possible that the compound crystallises in a pseudo-hexagonal orthorhombic space group. Additionally, IR and Raman spectra of all compounds were recorded.

Crystals of hexafluoropropene, C 3 F 6 , melting point 115 K, were obtained by low temperature crystallization on the diffractometer in a cold N 2 gas stream. The crystal structure was solved based on a dataset of 1347 reflections ((sinθ/λ max = 0.66 Å -1 ) measured at 95 K. C 3 F 6 crystallizes in the triclinic space group P-1 (No. 2), a = 6.072(6) Å, b = 6.233(5) Å, c = 6.569(6) Å, α = 92.33(5)°, β = 108.75(7)°, γ = 94.31(5)°, V = 234.2(6) Å 3 , Z = 2, R1 = 0.050 for 729 reflections ( F o 2 > 2σ(F o 2 )). The molecular structure has almost C s symmetry with all atoms in a common plane except two fluorine atoms of the CF 3 group. The fluorine atom of the CF 3 group in the molecular plane is in trans position to the fluorine atom bonded to the central carbon atom. The geometrical parameters obtained from X-ray data are compared to those from an earlier gas phase electron diffraction study and, moreover, to those, obtained by theory from different high level ab initio calculations which were carried out as well for hexafluoropropene as for propene itself.

The results of band structure (extended Hückel procedure) and cohesive energy calculations of spinel-type chromium oxides and sulfides, and of lithium vanadium chloride were presented. They confirm that the D 3d to C 3v symmetry reduction of the octahedrally coordinated metal ions (Cr, V) of the respective oxides and of Li 2 VCl 4 (space group F-43m instead of Fd-3m) is caused by relatively strong metal-metal bonds with crystal orbital overlap populations (COOP) up to 10% of those of the metal-oxygen bonds despite M-M distances ≅300pm. In the case of spinel-type chromium sulfides, the Cr-Cr interactions are antibonding in nature and, hence, no symmetry reduction occurs.

The new mineral biehlite was found in the Tsumeb mine, Namibia and has the composition Sb 1.79 As 0.21 MoO 6 . It crystallizes monoclinic, space group C2/c with lattice paramters a=18.076(5)Å, b= 5.920(5)Å, c=5.083(5)Å, β= 96.97(1)°. Refinement led to a final R(F) value of 0.065 for 679 symmetrically independent reflections. [MoO 6 ] 6- octahedra share common edges and form zigzag chains which run parallel to the c direction. The octahedra are flanked by a [Sb 2 O 2 ] 2+ group on each side and infinite ribbons of composition [Sb 4 Mo 2 O 12 ] are obtained. Neighbouring ribbons are only held together by weak Sb-O bonds in the directions of a and b. The [MoO 6 ] 6- octahedra are strongly distorted with Mo-O distances ranging from 1.700(5) to 2.248(6) Å and O-Mo-O angles between 72.1(2)° and 105,3(4)°. The common O-O edge (2.492(8)Å) is shortened with respect to the remaining edges of the octahedra (2.704(8)Å-2.899(8)Å). The Sb-cation forms three short bonds with oxygen at distances slightly smaller than 2Å. Six further oxygen atoms are located at distances ranging from 2.910(6) to 3.470(6)Å. Calculated distance for the lone-pair is 1.18 Å. Part of the Sb 3+ is substituted by the smaller As 3+ ion. The relationship of the new mineral to similar structures – especially to the mineral stibivanite and the synthetic compound Sb 2 MoO 6 – are discussed.

The structure of LaAlO 3 has been investigated around the phase transition at T c ≈ 800 K by neutron powder diffraction in vacuum and by X-ray powder diffraction under nitrogen atmosphere as well as by a very high resolution synchrotron experiment in air. The results were analysed in frame of the Landau theory using the fluctuation-dissipation theorem to relate the susceptibility to the atomic displacement parameters. The room temperature structure is a rhombohedrally distorted perovskite structure, space group R-3c , which undergoes a transition to the ideal perovskite structure, space group Pm-3m , at high temperatures. The order parameter is a rotation of the O 6 -octahedron described by one x(O)-parameter. This parameter and the spontaneous strain (c/a-√6), as well as the relevant atomic displacement parameter U op 11 (O) in the order parameter system, show a critical behaviour in agreement with a second order phase transition. Although the critical exponents of the order parameter and strain show the expected coupling behaviour, there is a striking difference of the transition temperature: the metric becomes cubic roughly 30 K below the proper T c . This is related to spontaneous formation of domains imposing the average cubic symmetry via internal stresses.

The crystal and molecular structures of 2-methyl-1-methylamino-anthraquinone (I) and 1-methylphenylamino-anthraquinone (II) were studied by the X-ray single-crystal diffraction and the visible spectra of crystalline specimens and their solutions were recorded. The molecule I is closely planar, whereas in the molecule II the amino group is 58° rotated out of the plane of the anthraquinone skeleton. In both structures the molecules pack in stacks. The comparison of experimental and calculated (on the DFT and AM1 levels) molecular structures, together with the comparison of experimental and INDO/S-calculated electronic spectra, give the evidence that molecular conformations (especially for II) change upon transfer from the solid state to solutions, and the π-delocalisation throughout the whole molecule enhances in the solid state.

Crystalline ternary mixed aminopyridine complexes of cobalt(II) chlorides and cobalt(II) bromides have been prepared and analysed by means of thermal (TG, DTG, DTA) methods, UV/VIS spectroscopy, magnetic measurements and X-ray powder diffraction techniques. The symmetry and cell dimensions have been calculated using TREOR 90, a trial-and-error indexing program. One of the compounds, namely the [Co(3-NH 2 -pyridine) 4 Cl 2 ] complex has been studied also by X-ray single-crystal diffraction, yielding a propeller-shaped molecule with octahedral co-ordination, exhibiting both molecular and crystallographic inversion symmetry. Crystal data: C 20 H 24 N 8 Cl 2 Co, M w = 506.30, triclinic (P-1) unit cell with a = 7.684(1), b = 8.582(1), c = 9.966(2) Å, α = 73.17(1), β= 69.97(1), γ = 70.33(1)°, V c = 570.0(2) Å 3 and Z = 1. The structure model was refined to R = 0.027 and wR = 0.037 for 2680 reflections with I > 3σ(I).