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 w ith the tetragonal tungsten bronze structure (TTB). The lattice consta nts 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 measurement s indicate cerium in the +III oxidation state. Both crystal structures of Nb and Ta phases have been determined and refined to reliability f actors 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 pentag onal 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 m icroscopy at high resolution (HREM) establishes the remarkable regular ity of the lattice, making it possible to precisely localize the vario us metals Nb, Ce, and K. (C) 1996 Academic Press, Inc.