title: A pseudo-tetragonal tungsten bronze superstructure: a combined solution of the crystal structure of K6.4(Nb,Ta)(36.3)O94 with advanced transmission electron microscopy and neutron diffraction.
abstract: The crystal structure of the K6.4Nb28.2Ta8.1O94 pseudo-tetragonal tungsten bronze-type oxide was determined using a combination of X-ray powder diffraction, neutron diffraction and transmission electron microscopy techniques, including electron diffraction, high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), annular bright field STEM (ABF-STEM) and energy-dispersive X-ray compositional mapping (STEM-EDX). The compound crystallizes in the space group Pbam with unit cell parameters a = 37.468(9) Å, b = 12.493(3) Å, c = 3.95333(15) Å. The structure consists of corner sharing (Nb,Ta)O6 octahedra forming trigonal, tetragonal and pentagonal tunnels. All tetragonal tunnels are occupied by K(+) ions, while 1/3 of the pentagonal tunnels are preferentially occupied by Nb(5+)/Ta(5+) and 2/3 are occupied by K(+) in a regular pattern. A fractional substitution of K(+) in the pentagonal tunnels by Nb(5+)/Ta(5+) is suggested by the analysis of the HAADF-STEM images. In contrast to similar structures, such as K2Nb8O21, also parts of the trigonal tunnels are fractionally occupied by K(+) cations.
authors.affiliation: Electron Microscopy for Materials Research (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020, Antwerp, Belgium. joke.hadermann@uantwerpen.be babaryk@univ.kiev.ua.
authors.forename: Robert
authors.initials: R
authors.lastname: Paria Sena
authors.affiliation: Faculty of Chemistry, Taras Shevchenko National University of Kyiv, Volodymyrska 64/13, 01601 Kyiv, Ukraine and Department of Physical and Inorganic Chemistry, Faculty of Chemistry, University of Oviedo, C/Julian Claveria 8, 33006 Oviedo, Spain.
authors.forename: Artem A
authors.initials: AA
authors.lastname: Babaryk
authors.affiliation: Department of Physical and Inorganic Chemistry, Faculty of Chemistry, University of Oviedo, C/Julian Claveria 8, 33006 Oviedo, Spain.
authors.forename: Sergiy
authors.initials: S
authors.lastname: Khainakov
authors.affiliation: Department of Physical and Inorganic Chemistry, Faculty of Chemistry, University of Oviedo, C/Julian Claveria 8, 33006 Oviedo, Spain.
authors.forename: Santiago
authors.initials: S
authors.lastname: Garcia-Granda
authors.affiliation: Faculty of Chemistry, Taras Shevchenko National University of Kyiv, Volodymyrska 64/13, 01601 Kyiv, Ukraine.
authors.forename: Nikolay S
authors.initials: NS
authors.lastname: Slobodyanik
authors.affiliation: Electron Microscopy for Materials Research (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020, Antwerp, Belgium. joke.hadermann@uantwerpen.be babaryk@univ.kiev.ua.
authors.forename: Gustaaf
authors.initials: G
authors.lastname: Van Tendeloo
authors.affiliation: Electron Microscopy for Materials Research (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020, Antwerp, Belgium. joke.hadermann@uantwerpen.be babaryk@univ.kiev.ua.
authors.forename: Artem M
authors.initials: AM
authors.lastname: Abakumov
authors.affiliation: Electron Microscopy for Materials Research (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020, Antwerp, Belgium. joke.hadermann@uantwerpen.be babaryk@univ.kiev.ua.
authors.forename: Joke
authors.initials: J
authors.lastname: Hadermann