SYNTHESIS AND CRYSTAL-STRUCTURE DETERMINATION OF TL8NB27.2O72 USING TEM AND SINGLE-CRYSTAL X-RAY-DIFFRACTION

Am investigation of the Nb-Tl-O system has allowed the synthesis of a new phase, Tl8Nb27.2O72. This phase is structurally and chemically clo se to a Tl-Nb-O phase previously described as a builtup hexagonal bron ze with a approximate to 7.4 Angstrom, b approximate to 13 Angstrom, c approximate to 7.6 Angstrom, and an orthorhombic C-type lattice, The electron diffraction study evidences a doubling of the c parameter wit h regard to the C cell, resulting in an orthorhombic I-type lattice wi th, in addition, the observation of a second superstructure and twinne d microdomains, A structural determination of the mean orthorhombic I- type cell was successfully carried out by combining energy dispersion spectroscopy (EDS) and single-crystal XRD studies and leads to a = 7.5 34(7) Angstrom, b = 12.992(12) Angstrom, c = 15,555(12) Angstrom, Z = 8, space group Im2m, R = 0.097, and R-w = 0.099 from 995 independent r eflections. A chemical formula for this thallium niobate, Tl8Nb27.2O72 , consistent with EDS results, was deduced by structure refinement, HR EM investigations on this phase clearly show irregular occupancy of th e supplementary crystallographic positions by niobium atoms together w ith a monocommensurate modulated structure and twinned microdomains, ( C) 1998 Academic Press.