Abstract
The crystal structure of Cu1.1Ta11O26.2F4.8 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) Å (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 KNb6O15F which has 50% of the five-sided tunnels transformed into pentagonal bipyramids. The composition of the framework is CuTa11(O,F)31, with Cu in square planar coordination linking to two pentagonal TaX7 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.
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