On the Arrangement of Pentagonal Columns in Tetragonal Tungsten Bronze-Type Nb18W16O93

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The evaluation of HAADF-STEM images of a sample with the composition Nb18W16O93 provided new insights into its real structure. The basic structure comprises an intact octahedral framework of the tetragonal tungsten bronze (TTB) type. The partial occupation of the pentagonal tunnels (PT) by metal–oxygen strings determines the oxygen-to-metal ratio (O/ΣM with M = Nb,W). For a large area, the O/ΣM was determined to be 2.755, which is bigger than the value of Nb18W16O93, which is O/ΣM = 2.735. To a large extent, the three-fold TTB superstructure of Nb8W9O47 with a high oxygen content is present (O/ΣM = 2.765). In addition, a new four-fold TTB superstructure was found in small domains. Nb12W11O63 with an O/ΣM = 2.739 obviously accommodates part of the sample’s metal excess compared to the stable phase Nb8W9O47.