Margareta Sundberg

Structure projection retrieval by image processing of HREM images taken under non-optimum defocus conditions.

A direct method for retrieval of the projected potential from a single HREM image of a thin sample is presented. Both out-of-focus and astigmatic images can be restored. The defocus and astigmatism values are first determined from the Fourier transform of the digitised HREM image. Then a filter is applied which reverts the phases of those Fourier components which have been reversed by the Contrast Transfer Function (CTF). The method has been incorporated into the CRISP image processing system. It can be applied on any sample, crystalline or amorphous. From thin crystalline areas the projected symmetry can be determined and a further improvement achieved by imposing the symmetry exactly. This compensates for the effects of crystal tilt. Five HREM images of a thin crystal of K(8-x)Nb(16-x)W(12+x)O80 (x = 1), taken with different defocus and astigmatism values, were processed. Only one, taken near Scherzer defocus, was directly interpretable before image processing. After processing, all images showed the projected potential of the structure. Using data to 2.77 angstroms resolution, all heavy (Nb/W) atom positions were found in every image, within on average 0.15 angstroms of the positions determined by single crystal X-ray diffraction. In the HREM images taken under non-optimum defocus conditions, also the potassium atoms in the tunnels of the structure were found.

New complex structures in the cesium-niobium-tungsten-oxide system revealed by HREM

Abstract Two new compounds in the Cs-Nb-W-O system, Cs x (Nb, W) 5 O 14 and Cs x (Nb, W) 27 O 73 , were synthesized and characterized by the high-resolution electron microscopy technique. The Cs x (Nb, W) 5 O 14 phase, with slightly variable composition, crystallizes in the orthorhombic system. The unit cell parameters are a = 27.1 A, b = 21.6 A and c = 3.94 A and the space group is P2 1 2 1 2. The structure was deduced from HREM images and verified by theoritical image calculations. Simulated images of different structure models showed that it was impossible to determine the amount of Cs present in the 7-sided tunnels from the contrast in the HREM images. The Cs content and the Nb/W ratio were obtained by EDS analysis. The Cs 0.7 Nb 2.7 W 2.3 O 14 structure can be described as composed of an arrangement of MO 6 octahedra and MO 7 pentagonal bipyramids (M = Nb, W) in such a way that 6- and 7-sided tunnels are formed in which the cesium ions are accomodated. The framework of Cs x (Nb, W) 5 O 14 is another example of a two-dimensionally complex structure appearing in the alkali-transition-metal-oxide systems. The Cs x (Nb, W) 27 O 73 phase was identified from electron diffraction patterns. The structure has tetragonal symmetry with unit cell dimensions a = 27.4 A and c = 3.96 A, space group P4/mbm, and it is isotypic with that of ”Rb 3 Nb 54 O 146 ”.

Structure of a high-pressure phase of vanadium pentoxide, β-V2O5

A high-pressure phase of vanadium pentoxide, denoted β-V2O5, has been prepared at P = 6.0 GPa and T = 1073 K. The crystal structure of β-V2O5 has been studied by X-ray and neutron powder diffraction, and high-resolution transmission electron microscopy. The V atoms are six-coordinated within distorted VO6 octahedra. The structure is built up of quadruple units of edge-sharing VO6 octahedra linked by sharing edges along [010] and mutually connected by sharing corners along [001]. This arrangement forms layers of V4O10 composition in planes parallel to (100). The layers are mutually held together by weak forces. β-V2O5 is metastable and transforms to α-V2O5 at 643–653 K under ambient pressure. Structural relationships between β- and α-V2O5, and between β-V2O5 and B-Ta2O5-type structures are discussed. The high-pressure β-V2O5 layer structure can be considered as the parent of a new series of vanadium oxide bronzes with cations intercalated between the layers.

Structure determination and correction for distortions in HREM by crystallographic image processing

Abstract A heavily distorted high resolution electron microscopy (HREM) image of a cesium-niobium oxide-fluoride sample was reconstructed by crystallographic image processing (CIP). In the reconstructed image, showing 4bm symmetry, all metal atoms were clearly seen with correct contrast, whereas in the original micrograph the fourfold symmetry was lost and only some areas could be interpreted in terms of an atomic arrangement. Defocus, astigmatism and electron beam and crystal tilt were evaluated by CIP, and it was found that the distortion in this case was mainly due to beam tilt and crystal tilt. After correcting for the distortions, a calculated map was compared to the structure of an apparently isomorphous compound ∼ TINb 7 O 18 , previously solved by X-ray diffraction. The atomic positions agreed to within ±6 pm. A simulated image with electron beam tilt was in good agreement with the experimental one.

The “pentagonal column” as a building unit in crystal and defect structures of some groups of transition metal compounds

Abstract The pentagonal column (PC) contains MX 7 bipyramids that share their equatorial edges with five MX 6 octahedra. Such groups are linked by their X vertices to form the one-dimensional infinite PC. This structure element appears rather frequently in transition metal oxides and related compounds. In the article use is made of PCs and aggregates of PCs to describe crystal structures and structural defects of such compounds.

The structure of LiCu2O2 with mixed-valence copper from twin-crystal data

The structure of LiCu2O2 was solved using two sets of X-ray diffraction twin-crystal data. Extended twinning creates virtual tetragonal symmetry. The compound crystallizes in Pnma (62) with unit-cell parameters a = 5.72 A, b = 2.86 A and c = 12.4 A with a certain homogeneity range. The structure consists of LiCuIIO2 layers interleaved by layers of CuI connected to oxygen in an almost linear coordination. Lithium and copper(II) have five oxygen neighbours in pyramidal arrangements that run as parallel bands through edge connections. Electron diffraction was used for characterizing the twinning.

Structure of UMoO5 studied by single-crystal X-ray diffraction and high-resolution transmission electron microscopy

A combination of X-ray diffraction and high-resolution transmission electron microscopy (HRTEM) has been used to study the crystal structure of molybdenum uranium pentoxide, UMoO 5 , obtained by hydrothermal and ceramic methods. Crystal data: M r = 414.0, orthorhombic, Pbaa (number 54), a=12.746(1), b = 7.3494 (7), c = 4.1252 (2) A, V = 386.4 (1) A 3 , Z = 4, D x = 7.116 Mgm -3 , R = 0.037 for 723 reflections. The structure of UMoO 5 is related to that of UVO 5 . Both are built up by slabs of pentagonal UO 7 bipyramids with slabs of MO 6 octahedra in-between. They differ in symmetry due to different types of displacement of the M atoms from the ab plane. The HRTEM study revealed a few defect regions in the UMoO 5 crystals prepared by ceramic methods. Energy-dispersive spectroscopy (EDS) analyses indicate a slight excess of uranium in such crystals. Hypothetical models of defect regions are given.

Structure and “oxidation behavior” of W24O70, a new member of the {103} CS series of tungsten oxides

Abstract Reduced tungsten trioxide crystals WO 3− x , formed by vapor transport from a preparation with bulk composition WO −2.90 , have been studied by X-ray diffraction and electron microscopy. A single-crystal X-ray investigation showed the existence of the ordered {103} CS -structure W 24 O 70 , a new member of the homologous series W n O 3 n −2 . Electron diffraction patterns of crystal fragments, with a few exceptions, showed the presence of the W 24 O 70 phase (composition WO 2.917 ). Lattice images, however, indicated a fairly ordered {103} CS -phase, W 24 O 70 , intergrown with slabs of WO 3 giving gross compositions of the examined crystals in the range WO 2.93 WO 2.96 . The wide WO 3 slabs were probably formed by an oxidation process during the preparation.

Stacking disorder and polytypism in enargite and luzonite

Abstract Microstructures of enargite and luzonite (Cu3AsS4) were studied using high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED). Enargite and luzonite are intergrown at the atomic level in samples from Recsk, Hungary. Both minerals typically contain faults along the planes of their close-packed layers. Comparisons of electron micrographs and images simulated for several types of fault models indicate that the planar defects can be interpreted as stacking faults in the regular cubic (luzonite) or hexagonal (enargite) sequence of close-packed layers. In addition to disordered layer sequences, two long-period, rhombohedral polytypes–9R, (21)3 and 24R, (311111)3–occur in enargite. The presence of defect-free luzonite and enargite indicates that both minerals grew directly from the hydrothermal solution. The disordered structures represent transitional structural states between luzonite and enargite and probably reflect the effects of fluctuating conditions during hydrothermal deposition.

HREM studies of phases based on α-U3O8-type layers in the Cu2OTa2O5 system

Abstract The high-resolution electron microscopy (HREM) technique was used to characterize phases in the Cu 2 OTa 2 O 5 system. The structures are closely related. They contain edge-sharing pentagonal TaO 7 bipyramids, forming layers of α-U 3 O 8 type. These layers are either single or double, the latter linked by apex oxygens. The layers are interleaved by octahedral Ta sites and linearly coordinated Cu positions so that a three-dimensional network is formed. The symmetry is hexagonal (or trigonal). Varying sequences of single (S) and double (D) layers give different c -axis lengths. The phases Cu 5 Ta 11 O 30 and Cu 3 Ta 7 O 19 are well ordered, with SDSD and DD sequences, respectively. Other ordered sequences, (SS) in “Cu 2 Ta 4 O 11 ” and (SDSSDS) in “Cu 7 Ta 15 O 41 ,” were also observed. Defects and disorder sometimes occur.