Intergrowth Tungsten Bronzes Research Articles

Synthesis and characterization of AsO[(W,Mo)O3]13, a new (6)-intergrowth tungsten bronze (ITB)

Crystals of a new arsenic tungsten-molybdenum oxide, AsO[(W,Mo)O3]13, have been grown through vapour phase transport at high temperature (1123 ​K). The crystal structure was investigated by combining electron and single-crystal X-ray diffraction techniques, electron microprobe chemical analysis and X-ray absorption spectroscopy. Single-crystal XRD yielded the orthorhombic unit-cell parameters a ​= ​25.0895, b ​= ​7.3061, c ​= ​3.9089 ​Å. Although electron diffraction indicated a doubled translation along [001], superstructure reflections were undetectable with X-ray diffraction so that only the average structure (sub-cell) was determined. The most reliable structural model was obtained in the P222 space group (R1 ​= ​6.96%). It exhibits an octahedral framework which forms six-octahedra thick perovskite slabs (PTB) alternated to single hexagonal tungsten bronze (HTB) modules, indicating that the compound corresponds to a (6)-ITB phase. As3+ was found to be disordered on two off-centred, symmetry-related sites into the hexagonal channels with the typical pyramidal AsO3 geometry. This is in agreement with the results of the extended X-ray absorption fine structure that indicate an As–O bond distance of 1.77 ​Å. The ordered distribution of As within the tunnels, likely related to the doubling of periodicity along [001], could not be determined.

Synthesis and crystal structure of a series of stoichiometric (n)-ITB molybdenum-bronze oxides containing trivalent arsenic

Abstract A series of six new single crystals of fully stoichiometric As3+-bearing Mo-oxides and partially W-substituted Mo-oxides with formula As m O(Mo1− x W x O3) p (m = 1, 2; p = 5, 7, 9, 10 and 11 and 0 ≤ x ≤ 0.6) was successfully grown using vapor-phase transport in vacuo. The crystal structures were determined using single-crystal X-ray diffraction data. All these compounds exhibit acentric orthorhombic symmetry with Z = 2, and belong to the so-called (n)-ITB (intergrowth tungsten bronzes) family, with n = 2, 3, 4 and 5. The six (n)-ITB phases have the following formulae: (2)-AsMo5O16 (Pm 2a), (2)-As2Mo10O31 (Pma 2), (3)-AsMo7O22 (Pmn 21), (3)-As(Mo5.53W1.47)O22 (Pmn 21), (4)-As(Mo4.33W4.67)O28 (Pm 2a) and (5)-As(W6.63Mo4.37)O34 (Pmn 21). Their structures consist of vertex-sharing MO6 octahedral units (with M either Mo or Mo/W) connected so as to form three-dimensional frameworks. Such frameworks consist of perovskite tungsten bronzes (PTB) type slabs, from 2- to 5-octahedra wide, intergrown with single hexagonal tungsten bronzes (HTB) type slabs, stacked up to form pseudo-hexagonal tunnels along the a-axis. As3+ and additional oxygen atoms are located in off-center positions inside the tunnels, forming As–O bonds with peculiar arrangements. In particular, we obtained the first examples of structures where, besides the usual AsO3E distorted pyramidal geometry, As3+ adopts AsO4E coordination with a seesaw configuration.

Nanoscale building blocks in a novel lithium arsenotungsten bronze: Synthesis and characterization

We report on a novel compound Li3AsW7O25 obtained by solid-state reaction and characterized by diffraction and spectroscopic methods. The bronze-type compound crystallizes in the orthorhombic space group Pbca with a=724.38(3)pm, b=1008.15(4)pm, c=4906.16(17)pm and Z=8. The structure is built up by chains of WO6 octahedra interconnected by AsO4 tetrahedra and WO6 octahedra forming a polyhedral arrangement as seen in intergrowth tungsten bronzes. The X-ray single crystal structure refinement allows solving the complex arsenotungstate framework. The powder neutron diffraction data analysis locates the lithium atoms. Thermal analysis showed that Li3AsW7O25 is stable up to its melting at 1135(3)K followed by a decomposition at 1182(5)K. The Kubelka–Munk treatment of the UV–vis spectrum revealed a wide band gap in the range of 2.84–3.40eV depending on the presumed electron transition type.

Exit wave reconstruction from focal series of HRTEM images, single crystal XRD and total energy studies on SbxWO3+y(x∼ 0.11)

A new tungsten bronze in the Sb—W—O system has been prepared in a solid state reaction from Sb 2 O 3 , WO 3 and W metal powder. The average structure was determined by single crystal X-ray diffraction. Sb x WO 3+ y ( x ∼ 0.11) crystallizes in the orthorhombic space group Pm 2 1 n (no. 31), a = 27.8135(9) Å, b = 7.3659(2) Å and c = 3.8672(1) Å. The structure belongs to the ( n )-ITB class of intergrowth tungsten bronzes. It contains slabs of hexagonal channels formed by six WO 6 octahedra. These slabs are separated by three layers of WO 6 octahedra that are arranged in a WO 3 -type fashion. The WO 6 octahedra share all vertices to build up a three-dimensional framework. The hexagonal channels are filled with Sb atoms to ∼80% and additional O atoms. The atoms are shifted out of the center of the channels. Exit-wave reconstruction of focal series of high resolution-transmission-electron-microscope (HRTEM) images combined with statistical paramäeter estimation techniques allowed to study local ordering in the channels. Sb atoms in neighbouring channels tend to be displaced in the same direction, which is in agreement with total energy calculations on ordered structure models, but the ratio of the occupation of the two possible Sb sites varies from channel to channel. The structure of Sb x WO 3+ y exhibits pronounced local modulations.

The Superstructure of Lead Tetragonal Tungsten Bronze

Electron-optical studies of the superstructure of lead tetragonal tungsten bronze (lead-TTB) are presented. Samples were synthesized for a range of lead compositions and synthesis conditions. For lead-TTB synthesized for very short reaction times, compositional analysis combined with electron diffraction revealed these specimens to also contain significant levels of an intergrowth tungsten bronze (ITB) phase. It was proposed that ITB was formed during the reaction as an intermediate between the tungsten oxide reagent and lead-TTB. Electron-diffraction investigations of lead-TTB over a range of specimen compositions determined that the large majority of crystallites examined exhibited a well-ordered 2√2 a TTB×√2b TTB×2 c TTB supercell periodicity, although evidence of a larger a-axis repeat was also observed. No evidence for a variation in the supercell with lead composition was observed. A model for the supercell was generated from consideration of stacking sequences of correlated ordered arrays of lead and tilted octahedra and it was demonstrated that larger superstructures could be generated using alternative stacking sequences.

Low Members of the Intergrowth Tungsten Bronze Family by Partial Substitution of Molybdenum for Tungsten

Potassium and cesium intergrowth tungsten bronzes AxMoyW1−yO3 with x≲0.25 and y≲0.65 have been prepared. By substitution of molybdenum for tungsten the synthesis temperature can be decreased to 600°C and new phases of the structure family prepared. The ITB members designated (1, 3) and (1, 1, 3) have been prepared in relatively phase-pure form, while individual crystals of (1, 2) and (1, 1, 2) have been identified by electron microscopy. These four phases have not been found previously in the ITB bronze systems.

High-Pressure Tungsten Bronzes, RExWO3 with RE=La and Nd, Studied by X-Ray Diffraction and Electron Microscopy

Rare earth tungsten bronzes, La0.1WO3 and NdxWO3 with x=0.1 and 0.2, have been prepared by solid state reaction at P=30 kbar and T=1320–1620 K and structurally characterized by X-ray diffraction and electron microscopy. The hexagonal tungsten bronze (HTB) structure was found in all samples, whereas intergrowth tungsten bronzes of (n)-ITB and (1, n)-ITB structure types have only been observed in samples prepared at T>1500 K. The ITB structures can be considered as an intergrowth of slabs of WO3-type, n octahedra wide, and slices of HTB-type. In the (n)-ITB family the slices of HTB-type are one hexagonal tunnel row wide, whereas they are two hexagonal tunnel rows wide in the (1, n)-ITB phases. Members of the (n)-ITB family were frequently observed in the high-temperature samples. The LaxWO3 bronzes, (2)-ITB, (3)-ITB, (1, 2)-ITB, and HTB, are new. In the NdxWO3 system a perovskite tungsten bronze (PTB) of composition Nd≈0.1WO3 and a (3)-ITB bronze were found in addition to the previously reported high-pressure phases (2)-ITB and HTB. The bronze structures were well-ordered, but a few exceptions were observed. A couple of defects are described along with proposed defect models. Microanalyses show that the hexagonal tunnel sites in the ITB and HTB structures are less than half-filled with RE atoms. The relationships between the RExWO3 bronzes and the alkali tungsten bronzes, AxWO3, are discussed.

Structural Studies of Calcium Tungsten Bronzes, CaxWO3, Formed at High Pressure

The structure of a new calcium intergrowth tungsten bronze, CaxWO3, prepared at P = 80 kbar and T = 1670 K, has been studied by high-resolution transmission electron microscopy and supplementary techniques. The unit-cell dimensions, determined from X-ray powder data, are: a = 10.1500(9), b = 7.4232(7) and c = 3.7915(4) Å. The crystal structure is of (n)-ITB type with n = 2. It is isotypic with those recently reported for the RExWO3 bronzes (RE = Pr and Nd) prepared at P = 50 kbar. Combined electron diffraction and energy-dispersive X-ray spectroscopy (EDX) studies of thin crystallites showed that the hexagonal tunnels were less than half-filled with Ca atoms (x ≈ 0.07 ∓ 0.01). Electron diffraction patterns revealed ordered superstructures of (2)-ITB as well as an order—disorder transformation under the intense electron beam. A calcium bronze (Ca≈0.12WO3) with an hexagonal tungsten bronze structure (HTB) was also observed in the sample.

Intergrowth Tungsten Bronze Structures of PrxWO3, Formed at 50 kbar: An HRTEM Study

New praseodymium intergrowth tungsten bronzes, PrxWO3, of (n)-ITB structure type have been prepared by solid state reaction at P=50 kbar and T=1570 K. The ordered members n=2, 3, and 4 were identified by high-resolution transmission electron microscopy. The structure of (2)-ITB was deduced from an electron micrograph, and the unit cell parameters were determined from the X-ray powder data: a=10.17 Å, b=7.418 Å, and c=3.790 Å. The praseodymium content (0.04≤x≤0.1), corresponding to less than half-filled hexagonal tunnels in the ITB-structures, was found by combining electron diffraction and microanalysis studies of the same crystal fragments. An ordered superstructure, designated (2)5(3)3-ITB, was also observed.

An X-Ray and Electron Diffraction Study of the Intergrowth Tungsten Bronze Sb0.16WO3

The average structure of the orthorhombic antimony tungsten bronze, Sb0.16WO3, has been resolved by three-dimensional single-crystal X-ray diffraction (space groupPmmm,aav= 7.4261(7),bav=10.2004(15),cav=3.8061(7) Å, andZ=5). The structure refinement based on 377 observed unique reflections withI>2σ(I) (3493 totally measured reflections) converged toR=0.064. The structure is of the intergrowth tungsten bronze type (ITB), consisting of slabs of hexagonal tungsten bronze structure (HTB) interchanged with slabs of perovskite-like WO3structure (PTB) along [010]. The HTB slabs are one hexagonal tunnel thick and the PTB slabs are two WO6octahedra thick. The Sb atoms are distributed over two positions inside the hexagonal tunnels. Electron diffraction observations revealed a superstructure withas=2aav,bs=2bav, andcs=4cav, which may be attributed to an ordering of the Sb atoms. A possible structural model is proposed.

`Inverted Twinning' in Intergrowth Tungsten Bronzes

A new type of twinning occurring in intergrowth tungsten bronzes (ITB) is described, revealed by high-resolution electron microscopy. Across the twin boundary the two structure elements of hexagonal tungsten bronze- and tetragonally distorted ReO3-types are interchanged and grow in strict geometrical relationship to each other. The structure is thus `inverted' and in the general case the two `twin' parts represent different members of the structure family. Some members remain invariant upon inversion, however. This defect is most often seen as ribbons in an ITB matrix in Mo-doped samples Cs x Mo y Wl−y O3, which require a lower synthesis temperature than pure tungsten bronzes. They may be frozen-in stages of a slow ordering process. A similar type of twinning might be found in other intergrowth structures.

Electronic properties of low-dimensional oxides Te 4Mo 20O 62 and VMoO 5

Crystals of some new low-dimensional compounds, Te 4Mo 20O 62 and VMoO 5, have been grown by gas phase transport method. From X-ray analysis the crystal structure of Te 4Mo 20O 62 was determined to be of the intergrowth tungsten bronze type. The crystal of VMoO 5 is a monoclinic plate-like structure. Te 4Mo 20O 62 is a semiconductor and the electrical resistivity shows a big two-dimensional anisotropy. It was also found to have a remarkable current dependent non-linear behavior. The magnetic susceptibility of VMoO 5 showed a metamagnetic behavior and an antiferromagnetic phase transition below 40 K.

Hrtem studies of two new (Nd,Ca) xWO 3 bronzes synthesized under high pressure conditions

Two new (Nd,Ca) x WO 3 bronzes have been prepared by solid state reaction at P = 50 kbar and T ≈ 1520 K. The compounds were studied by X-ray powder diffraction and high resolution transmission electron microscopy. One phase was identified to be of hexagonal tungsten bronze type, while the other one was found to have the intergrowth tungsten bronze structure (2)-ITB. Disorder in terms of varying widths of the HTB and WO 3 slabs was sometimes observed. HTB tunnel rows terminating in the (2)-ITB structure were also seen. The neodymium and calcium contents in the two phases were confirmed by EDS analysis.

The crystal chemistry of the higher tungsten oxides

The crystal chemistry of binary and ternary tungsten oxides with overall compositions between MO 3 and approximately MO 2 is reviewed. The chapter focuses primarily upon the reduced oxides and their fully oxidised analogues and descriptions of the structures have been employed which stress the linkage of component metal oxygen polyhedra. The stable form of the parent oxide WO 3 consists of corner-linked WO 6 octahedra. Reduction by way of the addition of metal atoms which can occupy the cage sites between these octahedra leads to the formation of the perovskite structure tungsten bronzes. Slight reduction of the binary oxide with medium sized metal atoms produces compounds containing units of edge-shared octahedra. These materials form the crystallographic shear (CS) phases. At greater degrees of reduction with similar metals pentagonal bipyramidal WO 7 polyhedra are utilised to form a variety of pentagonal column (PC) phases. This group includes the tetragonal tungsten bronze (TTB) structure and derivatives thereof. Low temperature preparations have lead to the formation of two other WO 3 structure types based upon rings of corner-sharing octahedra, one of which is the hexagonal tungsten bronze framework and the other the pyrochlore framework. These can be considered as parent structures for the high temperature hexagonal tungsten bronze (HTB) phases, the intergrowth tungsten bronze (ITB) phases and a number of ternary phases with structures related to that of pyrochlore. An extensive group of phases, the so-called ‘phosphate tungsten bronzes’, are ternary oxides containing phosphate groups, rather than bronzes. They are described in terms of the octahedral and tetrahedral polyhedra from which they are built up and their relationship to Mo 4O 11 structure indicated. A concluding section considers some aspects of the stability and reactivity of these phases.

On the interpretation of electron diffraction patterns from materials containing planar boundaries: the intergrowth tungsten bronzes

Materials containing planar boundaries are of general interest and complete understanding of their structures is important. When direct imaging of the boundaries by, for instance, high-resolution electron microscopy, is impracticable, details of their structure and arrangement may be obtained from electron diffraction patterns. Such patterns are discussed in terms of those from intergrowth tungsten bronzes as specific examples. Fourier-transform calculations for proposed structures have been made to establish, in conjunction with optical-diffraction analogues, the features of the far-field diffraction patterns. These results have been compared with diffraction patterns obtained experimentally by transmission electron microscopy. The aim of the study, to show that the arrangement of the boundaries in these complicated phases can be deduced from their diffraction patterns without the need for high-resolution imaging, has been achieved. The steps to be taken to make these deductions are set out.

A niobium phosphate bronze closely related to the ITB tungsten bronzes: K 7Nb 14+ xP 9− xO 60 ( x = 0.13)

A new niobium phosphate bronze K 7Nb 14+ x P 9− x O 60 has been isolated and its structure has been determined by X-ray diffraction from a single crystal. It crystallizes in the orthorhombic system, space group Pmma, with a = 36.883(3) Å, b = 10.603(1) Å and c = 6.4526(5) Å. The mixed framework of this bronze is closely related to the intergrowth tungsten bronzes (ITB): it consists of [Nb 3P 2O 13] ∞ layers deduced from the ITB [Mo 5O 15] ∞ layers observed in Sb 2Mo 10O 31 by replacing two octahedra out of five by PO 4 tetrahedra. Four such layers are stacked along a, sharing the corners of their polyhedra, forming [Nb 12P 8O 52] ∞ slabs connected through [NbP 2O 8] ∞ chains related to the perovskite and to the hexagonal tunnels. This host lattice determinates five types of tunnels of the brownmillerite and of the hexagonal tungsten bronzes, where the K + ions are located.

Scanning tunneling microscopy of the intergrowth tungsten bronze Rb0.05WO3

The surface structure of the intergrowth tungsten bronze, Rb0.05 WO3 was imaged at nanometer scale resolution using a scanning tunneling microscope in air. The images are explained within the context of the crystallographic properties of the material.

Imaging tunnel atoms in intergrowth tungsten bronzes

Crystals of caesium and rubidium intergrowth tungsten bronze (ITB) have been studied by high-resolution electron microscopy. The structure of these consists of slabs of WO3 type, intergrown with slabs of hexagonal tungsten bronze (HTB) type containing hexagonal tunnels in which the alkali atoms are accommodated. In the images of thicker parts the hexagonal tunnels are clearly revealed. In the thin parts the HTB structure mostly appears as a hexagonal pattern of dots of equal intensity and the tunnels are not recognizable. This applies particularly to Cs ITB but also in many instances to Rb ITB. Image simulations, assuming known or estimated degrees of filling of the tunnels with alkali, show a clear difference in contrast at the tungsten and alkali positions, especially at certain focus settings. The discrepancy is thought to be due to surface reconstruction, whereby tungsten atoms are trapped in the tunnels or the surface layers are shifted, making the surface structure similar to that of WO3.⅓H2O.

The structures of intergrowth tungsten bronzes of Ba, Sn, Pb, and Sb

The structures of intergrowth tungsten bronzes ( ITB) of compositions Ba 0.04WO 3, Sn 0.04WO 3, Pb 0.04WO 3, Sn 0.18WO 3, and Sb 0.25WO 3 have been deduced from high-resolution electron microscope images. Both the Pb 0.04, Sn 0.04, and Ba 0.04, ITB phases consist of single rows of hexagonal tunnels occupied by Pb, Sn, or Ba atoms intergrown in a WO 3-like matrix. The Sb 0.25, ITB phase is composed of similar rows of Sb-containing single hexagonal tunnels, the centers of which are separated by a WO 3-like matrix only two octahedra in thickness. The structure of the Sn 0.18, ITB phase consists of double rows of hexagonal tunnels containing Sn atoms joined by a single strip of WO 3-like octahedra. The structures are compared with the structures of other known ITB phases and the nonstoichiometric behavior of these phases is discussed.

ChemInform Abstract: Studies of Potassium Tungsten Bronzes Containing Lithium or Sodium.

AbstractMyKxWO3 compounds with M: Li or Na have been prepared from the corresponding oxides in closed ampoules at 500‐850 °C. Intergrowth tungsten bronzes (ITB) containing significant amounts of Li or Na could not be prepared.