Abstract
Samples of composition K 1− x Nb 3O 8− x F x (0 ⩽ x < 1) were studied by high resolution electron microscopy and X-ray powder diffraction and showed a variety of tetragonal-tungsten-bronze(TTB)-related phases for values of 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 8W 9O 47 and the recently reported H-KNb 3O 8 structural types, two new examples of ordered PC arrangements, corresponding to the stoichiometries KNb 12O 29F 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.
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