Eberhard Schweda

Structural analysis of a potassium hollandite K1.35Ti8O16

Crystals of K{sub 1.35}Ti{sub 8}O{sub 16} have been investigated by single-crystal X-ray analysis, X-ray powder patterns, electron diffraction, and high-resolution electron microscopy. K{sub 1.35}Ti{sub 8}O{sub 16} is obtained in the form of dark blue tetragonal crystals of the space group I4/m having the hollandite structure with a = 1,018.8(2) pm and c = 296.61(7) pm. The refinement converges to a reliability factor of 0.025. The occupation of the T(2,2) tunnel sites in potassium is not statistical. Incommensurate one-dimensional superstructures were found by powder diffraction patterns as well as by electron diffraction. The multiplicity calculated from powder X-ray diffraction is m = 5.79 and that found by electron diffraction is m = 8.1; this is explained in terms of beam damage and loss of potassium within the T(2,2) sites. The final step in the decomposition is the formation of rutile from K{sub 1.35}Ti{sub 8}O{sub 16} when all the potassium atoms are lost in the tunnels.

Neutron powder investigation of praseodymium and cerium nitride fluoride solid solutions

Abstract The investigation of CeN x F 3−3 x ( a = 5.8027(2) A) and PrN x F 3−3 x ( a = 5.7723(1) A) ( x ⋍ 0.33 ) solid solutions, using neutron powder diffraction, revealed that nitrogen atoms and fluorine atoms are occupying the tetrahedral holes within the fluorite type structure and that additional fluorine interstitials are observed on positions x , x , x ((32 f ) in Fm 3 m ) with x = 0.416. Some of the normal fluorite positions in 0.25, 0.25, 0.25 are relaxed on x , x , x ( x = 0.321 for CeN x F 3−3 x and x = 0.331 for PrN x F 3−3 x ) positions but can be considered rather as slightly relaxed normal sites than as true interstitials. The results have been interpreted with [1:0:3] and [1:0:4] defect clusters within these anion-excess-fluorite related structures.

Electron Holography at Atomic Dimensions—Present State

Abstract An electron microscope is a wave optical instrument where the object information is carried by an electron wave. However, an important information, the phase of the electron wave, is lost, because only intensities can be recorded in a conventional electron micrograph. Off-axis electron holography solves this “phase problem” by encoding amplitude and phase information in an interference pattern, the so-called hologram. After reconstruction, a rather unrestricted wave optical analysis can be performed on a computer. The possibilities as well as the current limitations of off-axis electron holography at atomic dimensions are discussed, and they are illustrated at two applications of structure characterization of ϵ-NbN and YBCO-1237. Finally, an electron microscope equipped with a Cs-corrector, a monochromator, and a Mollenstedt biprism is outlined for subangstrom holography.

Reactions of zerovalent olefin complexes of platinum with carbon monoxide

Abstract The reaction of platinum(0) olefin complexes with carbon monoxide at room temperature in hydrocarbon solvents was monitored gasvolumetrically and shown to give an unstable amorphous platinum(0) carbonyl derivative characterized by a CO:Pt molar ratio of 2 under the best conditions (low temperature, atmospheric pressure). The product of the olefin substitution by CO may be regarded as an intermediate towards the formation of platinum metal, whose diffraction pattern was detected upon heating the carbonylated product at about 250°C. The carbonylation product is highly reactive at room temperature towards di-iodine or the bidentate tertiary phosphine dppe giving PtI2(CO)2 or Pt(dppe)2 , respectively, in substantially quantitative yields. The present findings suggest the following relative affinity for platinum(0): dppe>CO>olefin. Steric congestion of the olefin ligands preventing the platinum atoms from readily forming metal–metal bonds presumably explains the experimentally established isolation of platinum(0) olefin complexes by Stone and coworkers.

The fluorite-related anion-excess structure of CeN0.222O0.667F1.333: Ordering of defect clusters

This investigation presents the preparation of CeN0.222O0.667F1.333 by a solid-state reaction from a mixture of CeN:CeF3:CeO2 = 1:2:1.5 and its structural investigation. The samples were annealed at 900°C in platinum tubes for different times. The basic structure found by powder neutron diffraction is anion-excess fluorite-related. The unit cell is an orthorhombic distortion of the cubic fluorite cell and has the space group Abm2. The lattice constants are a = 577.71(2) pm, b = 572.76(5) pm, and c = 573.32(6) pm. The structure refined by Rietveld analysis shows that [1:0:2]- defect clusters are present. In samples prepared by longer annealing times an ordering of these clusters to larger aggregates, i.e., toward the vernier phases, was observed. This was deduced from full profile analysis without refining a structural model by comparing the instrumental resolution curves of several models.

ReNF4 · ReF5(NCl), ein Nitrido-Nitrenokomplex von Rhenium(VII)/ ReNF4 · ReF5(NCl), a Nitrido Nitrene Complex of Rhenium (VII)

Abstract ReNF4 · ReF5(NCl) is prepared by direct fluorination of ReNCl4 with fluorine between 80 °C and 130 °C. The red crystals are extremely sensitive to moisture. The complex is characterized by the IR spectrum and by an X-ray structural investigation. ReNF4 · ReF5(NCl) crystallizes orthorhombically in the space group Pnma with 4 formula units per unit cell and with the cell dimensions a = 1440, b = 848, c = 776 pm (419 observed, independent reflexions, R = 13.9%). The complex consists of the molecules ReNF4 and ReF5(NCl), which are linked by a linear asymmetric fluorine bridge. The bridging fluorine atom is in trans-position to the nitrido ligand Re -F - 228 pm) and to the nitreno ligand (Re -F = 159 pm and Re= N - Cl 164 pm) correspond to triple 188 pm). The Re ≡ N bond lengths Re= N bonds.

Nitrido-azido-Komplexe des Molybdäns(VI) Synthese und Kristallstruktur von MoN(N3)3(NC5H5) / Nitrido Azido Complexes of Molybdenum (VI) Synthesis and Crystal Structure of MoN(N3)3(NC5H5)

Abstract The explosive nitrido azido complexes MoN(N3)Cl2 · py and MoN(N3)3py are prepared by the reaction of MoCl4(py)2 with (CH3)3SiN3 in 1,2-dichloroethane. Both compounds hydrolyze quickly in moist air. After separation of the insoluble, black MoN(N3)Cl2 · py from the solution of MoN(N3)3py, the latter can be obtained in form of monoclinic, red crystals of the space group C2/c. Its structure consists of monomeric complexes, wherein the Mo atom has a square pyramidal coordination. The nitrido ligand occupies the apex and forms a strong multiple bond of 163.4 pm to the Mo atom. σ-Bonds of different strength exist between the Mo atom and the basal ligands: MO-N3 - 204.3 pm; Mo-py = 225.8 pm. The α-N atoms of the azido groups are sp2 hybridized, with their lone pair pointing away from the nitrido ligand. The pyridine ligand forms an angle of 61.6° to the basal plane of the coordination polyhedron.

Synthese und Kristallstruktur eines Triazido-nitrido-bipyridyl-Komplexes des Molybdäns(VI): MoN(N3)3(bipy) / Synthesis and Crystal Structure of a Triazido Nitrido Bipyridyle Complex of Molybdenum(VI): MoN(N3)3(bipy)

The reaction of MoCl4(bipy) with an excess of (CH3)3SiN3 in 1,2-dichloroethane results in the formation of a mixture of MoN(N3)3(bipy) and Mo(N2)Cl2(bipy). The latter is an insoluble, brownish compound of yet unknown structure. MoN(N3)3(bipy) is very soluble in 1,2-dichloroethane and easily hydrolysed. It can be crystallized from toluene in the form of explosive, red crystals of the space group P 21/n. The crystal structure is built up by monomeric complexes, in which the Mo atom has a distorted sixfold coordination. The three azido groups are located cis to the nitrido ligand, with their free electron pair at the α-N-atom pointing away from the closely neighbouring nitrido ligand. The bipyridine forms two bonds of quite different length (224,0 and 241,9 pm) with the Mo atom, as trans to the triply bounded nitrido ligand (Mo≡N = 164,2 pm) only a weak interaction is possible.

Neutron powder diffraction study and DFT calculations on the structure of Zr10Sc4O26

Abstract Powder neutron diffraction was used to investigate Zr10Sc4O26, the so called γ-phase in the zirconia scandia system. This fluorite related anion deficient compound with composition MX1.86 crystallizes in space group R-3 and lattice constants a = 952.7(3) pm and c = 1745.3(4) pm. Anion vacancies are inserted on the 6c sites in R-3. The neutron data as well as the periodic density-functional calculations performed count for a divacancy model. The paired defect is more stable by 29 kJ/mol than any model with unpaired vacancies including the model with the 1/2[111]F (index F = fluorite) vacancy separation along an empty anion cube.

Phase transformation of ammonium monomolybdate. The structure of the low temperature modification, (NH4)2[MoO4] (mP60, P21/a)

Abstract (NH4)2[MoO4] (mS60) undergoes on cooling a phase transition at 190 K. The investigation of this phase transition by means of time and temperature resolved X-ray powder diffraction showed that the phase transition appears on heating at a temperature of 250 K. From this hysteresis and the abrupt changes in the peak positions of the diffractograms it is assumed that this is a first order transition. A diffractogram of the low temperature modification was recorded between 10° and 100° in 2θ. The structure was determined using direct methods and was refined by Rietveld methods. The low temperature modification of (NH4)2[MoO4] (mP60, P21/a) crystallizes in space group P21/a (No. 14) and lattice constants a = 767.57(5) pm, b = 1120.53(7) pm, c = 712.64(4) pm and β = 115.727(2)°.