Cu1.1Ta11O26.2F4.8: a tetragonal-tungsten-bronze-related structure with square planar coordination of copper

The crystal structure of Cu1.1Ta11O26.2F4.8 was determined by single crysta l X-ray diffraction in combination with HRTEM. The unit cell is orthorhombi c with a=17.7128(8), b=17.1832(7) and c=3.9412(1) Angstrom (X-ray powder da ta), space group Cmmm, and R=0.0327 for 594 independent reflections. The st ructure is related to the tetragonal tungsten bronze type structure of KNb6 O15F which has 50% of the five-sided tunnels transformed into pentagonal bi pyramids. The composition of the framework is CuTa11(O,F)(31), with Cu in s quare planar coordination linking to two pentagonal TaX7 bipyramids. The ad ditional Cu atoms are located in the five-sided tunnels. The composition gi ven to the compound was established by the X-ray refinement. The HRTEM stud y showed planar faults, twin and grain boundaries in the crystals. EDS-anal ysis indicated a somewhat higher Cu-content. (C) 2000 Elsevier Science Ltd. All rights reserved.