TTB structure of K(4)Ce(2)M(10)O(30) (M=Nb or Ta): Crystal growth and joint X-ray and HREM studies

K4C2Nb10O30 and K4Ce2Ta10O30 phases have been synthesized respectively at 1200 and 1250 degrees C. Single crystals grown at high temperature, respectively, 1400 and 1650 degrees C, and X-ray studies have shown that these isostructural phases crystallize in the tetragonal system with the tetragonal tungsten bronze structure (TTB). The lattice constants are a = 12.537(1) and c = 3.8976(7) Angstrom for Nb phase and a = 12.515(2) and c = 3.888(1) Angstrom for Ta phase. Magnetic measurements indicate cerium in the +III oxidation state. Both crystal structures of Nb and Ta phases have been determined and refined to reliability factors down to R(Nb) = 0.048 (785 hkl) and R(Ta) = 0.032 (836 hkl). ln these highly stoichiometric TTB structures, the K atoms hh the pentagonal tunnels with a tricapped trigonal prism of coordination polyhedra of oxygens, and the Ce atoms, filling the tetragonal tunnels, are in almost perfect cubooctahedra (CN = 12). An investigation by electron microscopy at high resolution (HREM) establishes the remarkable regularity of the lattice, making it possible to precisely localize the various metals Nb, Ce, and K. (C) 1996 Academic Press, Inc.