Monica Lundberg

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 ”.

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.

Li-inserted NbO2F studied by high-resolution electron microscopy and X-ray powder diffraction

Chemical insertion of Li in NbO2F has been performed under various conditions. NbO2F prepared from Nb-metal and a mixture of HF(aq) and HNO3(aq) was used as starting material, since NbO2F made from Nb2O5 and HF(aq) was shown to be NbO2F · HF. The products, with the formula LixNbO2F (0 1.3 were single phase of a hexagonal structure type. Samples with x = 0.0, 0.3, 0.5, and 1.2 were studied by electron diffraction and high-resolution electron microscopy techniques. Crystals of specimen x = 0.3 were shown to consist predominantly of a cubic phase surrounded by a shell of hexagonal LiNbO3 structure type, while the crystals of Li1.2NbO2F seemed to be built up mainly of the hexagonal phase.

Structures of niobium and tantalum oxide fluorides containing lone-pair ions: I. Pb3M4O12F2 (M = Nb or Ta): A Structure with Recognizable Pyrochlore Units

The tetragonal structure of Pb 3 M 4 O 12 F 2 ( M = Nb or Ta) has been determined and refined from single crystal X-ray diffractometer data to an R value of 0.064. The unit cell parameters are a Nb = 12.6595(3) A and c Nb = 7.4524(4) A, and the space group is P 4 2 / mnm . The parameters for the isomorphous tantalum phase are very similar. The structure is built up of corner-sharing M (O,F) 6 octahedra in such a way that cavities and tunnels arise, which contain the divalent lead atoms. The coordination numbers at these sites are 5 + 3 and 8, respectively. Bond strength calculations suggest that some of the anion sites are occupied predominantly by fluorine. The relationships with the pyrochlore and fluorite structures are discussed.

The crystal structure of MgNb14O35F2

Abstract A phase analysis of the system MgF 2 Nb 2 O 5 has revealed the existence of the compound MgNb 14 O 35 F 2 . The crystal structure has been solved from three-dimensional Weissenberg data and refined by least-squares technique. The symmetry is monoclinic, space group C2 m and the unit cell dimensions: a = 20.628 A; b = 3.825 A; c = 19.098A and β = 107.75°. The structure contains metal-oxygen (fluorine) octahedra which are linked by corners to form blocks measuring 5 × 3 × ∞ octahedra. The blocks are mutually connected along planes of crystallographic shear by octahedra sharing edges. The structure represents the previously missing member n = 5 of a series of monoclinic block structures Me 3 n X 8 n −3 found in the TiO 2 Nb 2 O 5 system by Wadsley.

New tetragonal tungsten-bronze-related phases in the pseudobinary system KNb3O8-Nb3O7F studied by high resolution electron microscopy and X-ray powder diffraction

Abstract Samples of composition K 1− x Nb 3 O 8− x F x (0 ⩽ x x = 0, 0.25, 0.50, 0.75 and 0.875. The structures are composed of pentagonal columns (PCs) and octahedra in various arrangements so that the TTB octahedral framework is maintained and the potassium atoms are located in four- and five-sided tunnels. Besides the well-known Nb 8 W 9 O 47 and the recently reported H-KNb 3 O 8 structural types, two new examples of ordered PC arrangements, corresponding to the stoichiometries KNb 12 O 29 F 3 and K x Nb 11 (O,F) 29 , were found. Twinning, defects and intergrowth were frequently observed in the samples. However, the X-ray powder pattern showed characteristic features for each composition given above. A new system of notation of TTB-related phases is introduced.

Phase analysis studies on the system PbOPbF2Nb2O5NbO2F

Abstract Samples in the system PbOPbF 2 Nb 2 O 5 NbO 2 F prepared at 875 K and 1025 K were studied by X-ray single crystal and powder diffraction as well as electron diffraction methods. Six new compounds were characterized as Pb 0.74 Nb 10.62 (O,F) 30 (orth.), Pb 1.36 Nb 10.91 (O,F) 30 (tetr.), Pb 0.27 Nb(O,F.) 3.17 (hex.), Pb 2 Nb 3 O 7 F 5 (tetr.), Pb 0.75 NbO 3 F 0.5 (tetr.) and Pb 1− x Nb x (O,F) 2+ y (cub.)

Thermal decomposition of lithium-inserted NbO2F

Abstract The thermal decomposition of lithium-inserted NbO 2 F was studied by differential scanning calorimetry. Samples of Li x NbO 2 F ( O ⩽ x ⩽ 1.8) were heated to 800 and 950 K. The thermograms revealed that decomposition started within the temperature range 640–780 K followed by a second step at approximately 900 K. The products were characterized by X-ray powder diffraction and electron diffraction patterns and by high resolution electron microscopy. The following phases were obtained at 800 K: LiF, NbO 2 F, the low pressure form of Nb 3 O 7 F, and lithium-enriched forms of PNb 2 O 5 , NbO 2 and LiNbO 3 . At 950 K, NbO 2 F disappeared, and LiNb 3 O 8 and the high pressure form of Nb 3 O 7 F coexisted with the other phases obtained at 800 K. The formation of structures built up of approximately hexagonally close-packed anion arrangements is discussed, as well as the role of lithium as a stabilizing component.

Cu1.1Ta11O26.2F4.8: a Tetragonal-Tungsten–Bronze-related structure with square planar coordination of copper

Abstract The crystal structure of Cu 1.1 Ta 11 O 26.2 F 4.8 was determined by single crystal X-ray diffraction in combination with HRTEM. The unit cell is orthorhombic with a =17.7128(8), b =17.1832(7) and c =3.9412(1) A (X-ray powder data), space group Cmmm , and R =0.0327 for 594 independent reflections. The structure is related to the tetragonal tungsten bronze type structure of KNb 6 O 15 F which has 50% of the five-sided tunnels transformed into pentagonal bipyramids. The composition of the framework is CuTa 11 (O,F) 31 , with Cu in square planar coordination linking to two pentagonal Ta X 7 bipyramids. The additional Cu atoms are located in the five-sided tunnels. The composition given to the compound was established by the X-ray refinement. The HRTEM study showed planar faults, twin and grain boundaries in the crystals. EDS-analysis indicated a somewhat higher Cu-content.

The structure of Bi15Ti9Fe11O57 and related compounds derived by high-resolution electron microscopy

Abstract The structure of a complex two-dimensional supercell observed by high-resolution electron microscopy in a layered bismuthate has been deduced from structural chemistry principles. It is proposed that this superstructure should form the basis for a new type of homologous series of related structures. An n = 5 member of the series, with composition Bi18Ti9Fe14O66, has been synthesized and shown to have regions with the same two-dimensional superstructure.